All states have a Section 218 agreement with the federal government's Social Security Administration. For more information see Chapter 10 of this Social Security Handbook. This July 8, 2004 Social Security Handbook chapter 10, section 1002.1 defines what is an interstate instrumentality.
Miles City is a city in and the county seat of Custer County, Montana, United States. The population was 8,123 at the 2010 census. After the Battle of the Little Bighorn in 1876, the U.S. Army created forts in eastern Montana, including one where the north-flowing Tongue River flowed into the east-flowing Yellowstone River. The first fort was known as the Tongue River Cantonment or the Tongue River Barracks and was founded on August 27, 1876. A second, permanent fort was constructed on higher ground two miles to the west of the mouth of the Tongue and this became Fort Keogh.
Billings is located in the south-central portion of the state and is the county seat of Yellowstone County, which had a 2015 population of 157,048. The 2015 Census estimates put the Billings population at 110,263 the only city in Montana to surpass 100,000 people. The city is experiencing rapid growth and a strong economy; it has had and is continuing to have the largest growth of any city in Montana. Parts of the metro area are seeing hyper growth.