Acquisitions (Details) - USD ($) $ in Thousands		3 Months Ended								9 Months Ended	12 Months Ended
	Apr. 01, 2019	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Acquisitions
Revenues		$ 719,584	$ 706,918	$ 650,302	$ 538,473	$ 588,359	$ 594,536	$ 535,043	$ 464,941		$ 2,615,277	$ 2,182,879	$ 1,787,922
Consideration transferred:
Total consideration transferred											2,600	96,400	9,400
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill		$ 332,447				$ 235,182				$ 332,447	$ 332,447	$ 235,182	$ 200,584
Walker
Acquisitions
Revenues										$ 343,200
Consideration transferred:
Cash paid at closing	$ 178,000
Advance to former owners	20,500
Working capital adjustment	(7,809)
Notes issued to former owners	25,000
Tax equalization payment	202
Estimated fair value of contingent earn-out payments	19,500
Total consideration transferred	235,393
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	8,225
Other long-term assets	53
Property and equipment	4,970
Goodwill	96,686
Identifiable intangible assets	90,200
Lease right-of-use asset	9,195
Accounts payable	(20,220)
Accrued compensation and benefits	(974)
Billings in excess of costs and estimated earnings	(31,553)
Other current liabilities	(11,205)
Long-term lease liabilities	(7,143)
Total assets acquired and liabilities assumed	$ 235,393
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%