General - Accounts Receivable, Contract Assets and Contract Liabilities - Tabular Disclosure (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Summary of Significant Accounting Policies
Accounts receivable	$ 856,059	$ 799,803
Change in accounts receivable	$ 56,256
Change in accounts receivable (as a percent)	7.00%
Contract assets	$ 8,114	3,566
Change in contract assets	$ 4,548
Change in contract assets (as a percent)	127.50%
Contract liabilities - current	$ 7,287	6,772
Change in contract liabilities - current	$ 515
Change in contract liabilities - current (as a percent)	7.60%
Contract liabilities - long-term	$ 11,290	$ 11,073
Change in contract liabilities - long-term	$ 217
Change in contract liabilities - long-term (as a percent)	2.00%