Acquisitions (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended	9 Months Ended
	Apr. 01, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Acquisitions
Revenues		$ 706,918	$ 594,536		$ 1,895,693	$ 1,594,520
Consideration transferred:
Total consideration transferred					2,600
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill		$ 332,200		$ 332,200	$ 332,200		$ 235,182	$ 200,584
Walker
Acquisitions
Revenues				$ 203,100
Consideration transferred:
Cash paid at closing	$ 178,000
Advance to former owners	20,500
Working capital adjustment	(7,594)
Notes issued to former owners	25,000
Tax equalization payment	202
Estimated fair value of contingent earn-out payments	19,500
Total consideration transferred	235,608
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents	4,850
Billed and unbilled accounts receivable	92,309
Other current assets	8,587
Other long-term assets	53
Property and equipment	4,970
Goodwill	96,439
Identifiable intangible assets	90,200
Lease right-of-use asset	(10,495)
Accounts payable	(20,220)
Accrued compensation and benefits	(974)
Billings in excess of costs and estimated earnings	(31,553)
Other current liabilities	(11,278)
Long-term lease liabilities	(8,270)
Total assets acquired and liabilities assumed	$ 235,608
Cash flow discount rate	10.20%
Walker | Minimum
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash flow discount rate	8.50%
Walker | Maximum
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash flow discount rate	11.50%