Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant

  
related to any such award. As of December 31, 2021, the remaining aggregate number of shares of the Company's common stock authorized for issuance under the 2012 Plan was 3.6 million.   
The 2011 Plan permitted the awards of stock options and other stock awards. Stock awards, in the form of stock rights, were granted to members of the board of directors without payment to the Company.   
Stock Awards  
Stock awards include RPSRs, RSRs, and stock rights. The fair value of stock awards is determined based on the closing market price of the Company's common stock on the grant date. Compensation expense for stock awards is measured based on the grant date fair value and recognized over the vesting period, generally three years.   
For purposes of measuring compensation expense, the amount of shares ultimately expected to vest is estimated at each reporting date based on management's expectations regarding the relevant service or performance criteria.  
The Company issued the following stock awards in the years ended December 31, 2021, 2020, and 2019:  
Restricted Performance Stock Rights - For the year ended December 31, 2021, the Company granted approximately 0.2 million RPSRs at a weighted average share price of $180.06. These rights are subject to cliff vesting on December 31, 2023. For the year ended December 31, 2020, the Company granted approximately 0.1 million RPSRs at a weighted average share price of $229.06. These rights are subject to cliff vesting on December 31, 2022. For the year ended December 31, 2019, the Company granted approximately 0.1 million RPSRs at a weighted average share price of $210.24. These rights were fully vested as of December 31, 2021. All of the RPSRs are subject to the achievement of performance-based targets at the end of the respective vesting periods and will ultimately vest between 0% and 200% of grant date value.  
Restricted Stock Rights - Retention stock awards are granted to key employees primarily to ensure business continuity. In 2021, the Company granted approximately 31,400 RSRs at a weighted average share price of $187.59, with cliff vesting one to three years from the grant date. In 2020, the Company granted less than 1,000 RSRs at a weighted average share price of $192.26, with cliff vesting two to three years from the grant date. In 2019, no retention stock awards were granted. As of December 31, 2021, approximately 29,800 RSRs were outstanding.   
For the year ended December 31, 2021, 0.1 million stock awards vested, of which less than 0.1 million were transferred to the Company from employees in satisfaction of minimum tax withholding obligations. For the year ended December 31, 2020, 0.1 million stock awards vested, of which less than 0.1 million were transferred to the Company from employees in satisfaction of minimum tax withholding obligations. For the year ended December 31, 2019, 0.3 million stock awards vested, of which approximately 0.1 million were transferred to the Company from employees in satisfaction of minimum tax withholding obligations.  
Stock Rights and Stock Issuances - The Company granted stock rights to its non-employee directors on a quarterly basis in 2021, with each grant less than 10,000 shares. All stock rights granted to non-employee directors are fully vested on the grant date. If a non-employee director has met certain stock ownership guidelines, the non-employee director may elect under the terms of the Amended and Restated Directors’ Compensation Policy and Amended and Restated Board Deferred Compensation Policy to receive their annual equity award for the following calendar year in the form of either shares of the Company’s common stock or stock units that are payable in the fifth calendar year after the year in which the annual equity award is earned, or, if earlier, upon termination of the director’s board service.  
Non-employee directors may also elect to receive their annual cash retainers in the form of stock units that become payable upon termination of the director’s board service. Non-employee directors who elect to receive their annual cash retainers in the form of stock units and have met their stock ownership guidelines may elect under the terms of the Amended and Restated Directors’ Compensation Policy and Amended and Restated Board Deferred Compensation Policy to receive stock units for the following calendar year that are payable in the fifth calendar year after the year in which the stock units are earned, or, if earlier, upon termination of the director’s board service.  
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