Contract Balances - Valuation And Qualifying Accounts Activity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)
Contract Balances
Allowance for Doubtful Accounts Receivable, Beginning Balance	$ 72,278
Current Period Provision for Expected Credit Losses	37,222
Write-offs Charged Against Allowance	(61,519)
Allowance for Doubtful Accounts Receivable, Ending Balance	$ 47,981