Business Segments (Narrative) (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($) segment $ / shares		Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) contract $ / shares
Segment Reporting Information [Line Items]
Number of reportable segments | segment									3
Income from construction operations	$ 90,680	$ 47,306	$ 54,815	$ (925)	$ 59,343	$ 49,072	$ 34,045	$ 37,017	$ 191,876		$ 179,477	$ 201,920
DILUTED EARNINGS PER COMMON SHARE | $ / shares	$ 0.98	$ 0.42	$ 0.49	$ (0.24)	$ 1.60	$ 0.47	$ 0.59	$ 0.27	$ 1.66		$ 2.92	$ 1.92
Building
Segment Reporting Information [Line Items]
Income from construction operations									$ 43,939	[1]	$ 34,199	$ 51,564	[2]
Civil
Segment Reporting Information [Line Items]
Income from construction operations									168,256	[1]	192,207	$ 172,668	[2]
Civil | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations									17,800
Income from construction operations, net of tax									$ 12,800
DILUTED EARNINGS PER COMMON SHARE | $ / shares									$ 0.25
Specialty Contractors
Segment Reporting Information [Line Items]
Number of projects | contract												2
Income from construction operations									$ 43,430	[1]	$ 18,938	$ 37,908	[2]
Specialty Contractors | Net Favorable Adjustment [Member]
Segment Reporting Information [Line Items]
Income from construction operations												3,000
Income from construction operations, net of tax												$ 1,800
DILUTED EARNINGS PER COMMON SHARE | $ / shares												$ 0.04
Specialty Contractors | Unfavorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations												$ (13,800)
Income from construction operations, net of tax												$ 8,100
DILUTED EARNINGS PER COMMON SHARE | $ / shares												$ (0.16)
Specialty Contractors | Favorable Adjustments [Member]
Segment Reporting Information [Line Items]
Income from construction operations												$ 14,000
Income from construction operations, net of tax												$ 8,200
DILUTED EARNINGS PER COMMON SHARE | $ / shares												$ 0.17

[1] During the year ended December 31, 2018, the Company recorded a charge of $17.8 million in income from construction operations (an after-tax impact of $12.8 million, or $0.25 per diluted share), which was primarily non-cash, as a result of the unexpected outcome of an arbitration decision related to a subcontract back charge dispute on a Civil segment project in New York that was completed in 2013.

[2] During the year ended December 31, 2016 the Company recorded net favorable adjustments totaling $3.0 million in income from construction operations (an after-tax impact of $1.8 million, or $0.04 per diluted share) for various Five Star Electric projects in New York in the Specialty Contractors segment. The net impact included material adjustments related to two electrical subcontract projects: a favorable adjustment of $14.0 million for a completed project (an after-tax impact of $8.2 million, or $0.17 per diluted share) and an unfavorable adjustment of $13.8 million for a project that was nearly complete (an after-tax impact of $8.1 million, or $0.16 per diluted share).