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Commitments and Contingencies - Legal (Details) $ in Thousands		1 Months Ended	3 Months Ended				9 Months Ended
	Feb. 25, 2015 USD ($)	Nov. 05, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2014 USD ($) lawsuit	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Commitments and contingencies
Gross profit			$ 106,505		$ 70,421		$ 222,484	$ 209,957
JCG \| North Texas Tollway Authority v. James Construction Group, LLC
Commitments and contingencies
Liability recorded on litigation	$ 17,000						$ 3,800
Expected remediation cost on settlement	22,400
Percentage of expected costs second defendant would pay			20.00%				20.00%
Percentage of expected costs Company would pay			80.00%				80.00%
Payment for remediation							$ 500
Accrual balance			$ 18,500				18,500
JCG \| North Texas Tollway Authority v. James Construction Group, LLC \| Maximum
Commitments and contingencies
Agreed payments by second defendant in expected remediation costs toward settlement	$ 5,400
Willbros Group, Inc
Commitments and contingencies
Number of lawsuits \| lawsuit						2
Disputed Receivables
Commitments and contingencies
Additional partial settlement			9,000
Receivable recorded relating to the project		$ 11,900	20,900			$ 32,900	20,900
Gross profit			6,200				6,200
Reserve			11,600				11,600
Receipts related to disputed receivable		$ 9,000		$ 12,000
Disputed Receivables \| Customer Liabilities
Commitments and contingencies
Reserve			17,900				17,900
Withdrawal liability for multiemployer pension plan
Commitments and contingencies
Payment of liability			4,300
Bonding
Commitments and contingencies
Bid and completion bonds issued and outstanding			$ 508,500				$ 508,500		$ 705,700