Contract Balances - Valuation And Qualifying Accounts Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Contract Balances
Allowance for Doubtful Accounts Receivable, Beginning Balance	$ 19,280	$ 16,966	$ 15,511
Current Period Provision for Expected Credit Losses	84,697	69,866	98,575
Write-offs Charged Against Allowance	(31,699)	(67,552)	(97,120)
Allowance for Doubtful Accounts Receivable, Ending Balance	$ 72,278	$ 19,280	$ 16,966