Goodwill and Other Intangible Assets (Rollforward of Other Intangible Assets) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Other Intangible Assets [Line Items]
Other intangible assets, gross						$ 354,000
Accumulated amortization						(184,600)
Other Intangible Assets [Rollforward]
Other intangible assets, net, beginning balance				$ 169,370
Additions from new business combinations				28,600
Measurement period adjustments	[1]			(300)
Amortization expense		$ (4,700)	$ (5,200)	(14,200)	$ (15,400)
Currency translation adjustments				1,400
Other intangible assets, net, ending balance		184,938		184,938
Customer Relationships and Backlog [Member]
Other Intangible Assets [Line Items]
Other intangible assets, amortizing, gross						224,400
Accumulated amortization						(170,000)
Other Intangible Assets [Rollforward]
Other intangible assets, net, amortizing, beginning balance				54,400
Additions from new business combinations, amortizing intangible assets				27,000
Measurement period adjustments, amortizing intangible assets	[1]			(300)
Amortization expense				(12,900)
Currency translation adjustments, amortizing intangible assets				100
Other intangible assets, net, amortizing, ending balance		68,300		68,300
Other Amortizing Intangible Assets [Member]
Other Intangible Assets [Line Items]
Other intangible assets, amortizing, gross	[2]					21,100
Accumulated amortization	[2]					(14,600)
Other Intangible Assets [Rollforward]
Other intangible assets, net, amortizing, beginning balance	[2]			6,500
Additions from new business combinations, amortizing intangible assets	[2]			1,600
Measurement period adjustments, amortizing intangible assets	[1],[2]			0
Amortization expense	[2]			(1,300)
Currency translation adjustments, amortizing intangible assets	[2]			0
Other intangible assets, net, amortizing, ending balance	[2]	6,800		6,800
Trade Names [Member]
Other Intangible Assets [Line Items]
Other intangible assets, non-amortizing		34,500		34,500		34,500
Other Intangible Assets [Rollforward]
Other intangible assets, non-amortizing, beginning balance				34,500
Additions from new business combinations, non-amortizing intangible assets				0
Measurement period adjustments, non-amortizing intangible assets	[1]			0
Currency translation adjustments, non-amortizing intangible assets				0
Other intangible assets, non-amortizing, ending balance		34,500		34,500
Pre-Qualifications [Member]
Other Intangible Assets [Line Items]
Other intangible assets, non-amortizing		75,300		74,000		$ 74,000
Other Intangible Assets [Rollforward]
Other intangible assets, non-amortizing, beginning balance				74,000
Additions from new business combinations, non-amortizing intangible assets				0
Measurement period adjustments, non-amortizing intangible assets	[1]			0
Currency translation adjustments, non-amortizing intangible assets				1,300
Other intangible assets, non-amortizing, ending balance		$ 75,300		$ 75,300

[1] Represents adjustments to preliminary estimates of fair value within the measurement period of up to one year from the date of acquisition.

[2] Consists principally of trade names and non-compete agreements.