Accounts Receivable, Net of Allowance, and Contract Assets and Liabilities (Schedule of Accounts Receivable, Net of Allowance and Contract Assets) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Receivables [Abstract]
Contract billings	$ 874,400	$ 805,000
Less allowance	(9,200)	(20,500)
Accounts receivable, net of allowance	865,235	784,488
Contract Assets [Abstract]
Retainage	271,500	287,700
Unbilled receivables	832,200	682,000
Contract assets	$ 1,103,726	$ 969,743