Accounts Receivable, Net of Allowance (Schedule of Accounts Receivable, Net of Allowance) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts Receivable, Net of Allowance
Contract billings	$ 683,900	$ 564,200
Retainage	323,100	268,600
Cost and earnings in excess of billings	599,200	331,600
Accounts receivable, gross	1,606,200	1,164,400
Less allowance for doubtful accounts	(8,200)	(8,400)	$ (7,700)
Accounts receivable, net	$ 1,598,015	$ 1,156,031