Business Combinations - Summary of Estimated Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Oct. 02, 2020	Mar. 06, 2020	Sep. 27, 2019	Jun. 12, 2019	Dec. 15, 2017
Assets
Goodwill	$ 5,639,091		$ 5,432,544
CH2M HILL Companies, Ltd.
Assets
Cash and cash equivalents					$ 315,200
Receivables					1,120,600
Prepaid expenses and other					72,700
Property, equipment and improvements, net					175,100
Goodwill					3,165,500
Identifiable intangible assets					416,700
Miscellaneous					530,800
Total Assets					5,796,600
Liabilities
Notes payable					2,200
Accounts payable					309,600
Accrued liabilities					787,400
Contract liabilities					260,800
Lease intangible liabilities					9,600
Long-term debt					706,000
Other deferred liabilities					659,000
Total Liabilities					2,734,600
Noncontrolling interests					(37,300)
Net assets acquired					3,024,700
CH2M HILL Companies, Ltd. | Customer relationships, contracts and backlog
Assets
Identifiable intangible assets					412,300
CH2M HILL Companies, Ltd. | Lease intangible assets
Assets
Identifiable intangible assets					$ 4,400
KeyW Holding Corporation
Assets
Cash and cash equivalents				$ 29,100
Receivables				79,100
Inventories, net				19,300
Prepaid expenses and other				2,400
Property, equipment and improvements, net				24,500
Deferred tax asset and other				37,800
Goodwill				615,600
Identifiable intangible assets				179,000
Total Assets				986,800
Liabilities
Accounts payable				8,300
Accrued liabilities				69,100
Short term debt				298,400
Other current liabilities				3,900
Other deferred liabilities				2,900
Total Liabilities				382,600
Net assets acquired				$ 604,200
John Wood Group
Assets
Cash and cash equivalents		$ 24,300
Receivables		75,900
Other current assets		5,200
Property, equipment and improvements, net		8,300
Goodwill		205,800
Identifiable intangible assets		80,000
Miscellaneous		19,400
Total Assets		418,900
Liabilities
Accounts payable		71,800
Long-term debt		29,200
Total Liabilities		101,000
Net assets acquired		$ 317,900