Goodwill and Intangible Assets (Changes in Carrying Amount of Goodwill) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Gross goodwill as of December 31, 2018			$ 1,072,991
Accumulated impairment as of December 31, 2018	$ (867,800)		(487,985)
Balance at beginning of period	205,143	$ 585,006
Goodwill impairment	0	(379,863)	0
Current year activity	0
Balance at end of period	205,143	205,143	585,006
Civil
Goodwill [Line Items]
Gross goodwill as of December 31, 2018			492,074
Accumulated impairment as of December 31, 2018			(76,716)
Balance at beginning of period	205,143	415,358
Goodwill impairment		(210,215)
Current year activity	0
Balance at end of period	205,143	205,143	415,358
Building
Goodwill [Line Items]
Gross goodwill as of December 31, 2018			424,724
Accumulated impairment as of December 31, 2018			(411,269)
Balance at beginning of period	0	13,455
Goodwill impairment		(13,455)
Current year activity	0
Balance at end of period	0	0	13,455
Specialty Contractors
Goodwill [Line Items]
Gross goodwill as of December 31, 2018			156,193
Accumulated impairment as of December 31, 2018			0
Balance at beginning of period	0	156,193
Goodwill impairment		(156,193)
Current year activity	0
Balance at end of period	$ 0	$ 0	$ 156,193