Accounts Receivable, Net of Allowance, and Contract Assets and Liabilities (Schedule of Accounts Receivable, Net of Allowance and Contract Assets) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Receivables [Abstract]
Contract billings	$ 889,500	$ 805,000
Less allowance	(12,800)	(20,500)
Accounts receivable, net of allowance	876,715	784,488
Contract Assets [Abstract]
Retainage	314,000	287,700
Unbilled receivables	644,100	682,000
Contract assets	$ 958,064	$ 969,743