Acquisitions - Additional Information (Details) $ in Thousands			3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Jun. 14, 2018 USD ($)	Apr. 03, 2018 USD ($)	Sep. 30, 2018 USD ($) Office	Sep. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) Office	Sep. 30, 2017 USD ($)	Dec. 31, 2018	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Common stock issued in acquisition						$ 321,075	$ 0
Revenue				$ 1,087,111		2,638,664	2,539,178
Acquisition and integration expenses			$ 9,334	0		44,030	0
Goodwill			244,696	53,799		244,696	53,799		$ 53,799
Amortization expenses			4,200			6,700
Amortization expenses remainder of 2018			4,600			4,600
Amortization expenses in 2019			16,200			16,200
Amortization expenses in 2020			11,300			11,300
Amortization expenses in 2021			8,700			8,700
Amortization expenses thereafter			12,100			$ 12,100
Federal statutory tax, Percent						21.00%			35.00%
Proforma Adjustments
Business Acquisition [Line Items]
Federal statutory tax, Percent									39.00%
Scenario, Forecast [Member] | Proforma Adjustments
Business Acquisition [Line Items]
Federal statutory tax, Percent								26.00%
Water [Member]
Business Acquisition [Line Items]
Goodwill			135,230	618		$ 135,230	618		$ 618
Materials [Member]
Business Acquisition [Line Items]
Goodwill			49,847	1,946		49,847	1,946		1,946
Specialty [Member]
Business Acquisition [Line Items]
Goodwill			$ 39,821	$ 31,437		$ 39,821	$ 31,437		$ 31,437
Layne Christensen Company [Member]
Business Acquisition [Line Items]
Acquisition purchase price	$ 349,840
Common stock issued in acquisition	321,000
Acquisition cash settled	28,800
Acquisition assumed liabilities	191,500
Number of offices | Office			52			52
Revenue			$ 129,100			$ 150,600
Net income (loss) before tax attributable to Granite			8,600			(6,700)
Acquisition and integration expenses			3,400			18,100
Decrease in property and equipment			(2,500)
Decrease in identifiable intangible assets			(2,300)
Increase in deferred income taxes			1,100
Increase in goodwill			3,700		$ 7,600
Decrease the investments in affiliates					$ (7,600)
Business consideration tangible assets acquired	430,015
Identifiable intangible assets	58,448
Business consideration intangible liabilities assumed	6,700
Goodwill	$ 185,228
Layne Christensen Company [Member] | Water [Member]
Business Acquisition [Line Items]
Goodwill			128,900			128,900
Layne Christensen Company [Member] | Materials [Member]
Business Acquisition [Line Items]
Goodwill			47,900			47,900
Layne Christensen Company [Member] | Specialty [Member]
Business Acquisition [Line Items]
Goodwill			8,400			8,400
LiquiForce Services Company [Member]
Business Acquisition [Line Items]
Acquisition assumed liabilities		$ 35,900
Business consideration tangible assets acquired		14,300
Identifiable intangible assets		10,900
Business consideration intangible liabilities assumed		8,500
Goodwill		$ 19,200
LiquiForce Services Company [Member] | Water [Member]
Business Acquisition [Line Items]
Goodwill			$ 19,200			$ 19,200