Construction Contracts - Net Position of Uncompleted Construction Contracts (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Contractors [Abstract]
Costs incurred on uncompleted construction contracts	$ 520,368	$ 333,744
Estimated earnings	18,070	10,936
Billings	(541,784)	(354,737)
Net position	(3,346)	(10,057)
Construction contract costs and estimated earnings in excess of billings	245	110
Billings in excess of construction contract costs and estimated earnings	$ (3,591)	$ (10,167)