Goodwill and Intangible Assets (Changes in Carrying Amount of Goodwill) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Gross goodwill as of December 31, 2020		$ 1,072,991
Accumulated impairment as of December 31, 2020		(867,848)
Balance at beginning of period	$ 205,143
Current year activity	0
Balance at end of period	205,143
Civil
Goodwill [Roll Forward]
Gross goodwill as of December 31, 2020		492,074
Accumulated impairment as of December 31, 2020		(286,931)
Balance at beginning of period	205,143
Current year activity	0
Balance at end of period	205,143
Building
Goodwill [Roll Forward]
Gross goodwill as of December 31, 2020		424,724
Accumulated impairment as of December 31, 2020		(424,724)
Balance at beginning of period	0
Current year activity	0
Balance at end of period	0
Specialty Contractors
Goodwill [Roll Forward]
Gross goodwill as of December 31, 2020		156,193
Accumulated impairment as of December 31, 2020		$ (156,193)
Balance at beginning of period	0
Current year activity	0
Balance at end of period	$ 0