In December 2017, 25% of the employees in Washington, D.C., were employed by a federal governmental agency. This is thought to immunize Washington, D.C., to national economic downturns because the federal government continues operations even during recessions. Many organizations such as law firms, defense contractors, civilian contractors, nonprofit organizations, lobbying firms, trade unions, industry trade groups, and professional associations have their headquarters in or near Washington, D.C., in order to be close to the federal government. The city of Rosslyn, Virginia, located across the Potomac River from D.C., serves as a base of operations for several Fortune 500 companies, due to the building height restrictions in place within the District of Columbia. In 2018, Amazon announced they would build "HQ 2" in the Crystal City neighborhood of Arlington, Virginia.