Accounts Receivable, Net of Allowance (Schedule of Accounts Receivable, Net of Allowance) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Accounts Receivable, Net of Allowance
Contract billings	$ 784,500	$ 564,200
Retainage	228,600	268,600
Costs and earnings in excess of billings	565,100	331,600
Accounts receivable, gross	1,578,200	1,164,400
Less allowance for doubtful accounts	(8,800)	(8,400)
Accounts receivable, net	$ 1,569,392	$ 1,156,031