Goodwill and Other Intangible Assets (Rollforward of Other Intangible Assets) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Other Intangible Assets [Line Items]
Other intangible assets, gross				$ 356,900
Accumulated amortization				(165,800)
Other Intangible Assets [Rollforward]
Other intangible assets, net, beginning balance		$ 191,142
Additions from new business combinations, other intangible assets		3,200
Measurement period adjustments, other intangible assets	[1]	(700)
Amortization expense		(4,900)	$ (4,100)
Currency translation adjustments, other intangible assets		(1,300)
Other intangible assets, net, ending balance		187,442
Customer Relationships and Backlog [Member]
Other Intangible Assets [Line Items]
Other intangible assets, amortizing, gross				223,000
Accumulated amortization				(152,400)
Other Intangible Assets [Rollforward]
Other intangible assets, net, amortizing, beginning balance		70,600
Additions from new business combinations, amortizing intangible assets		2,900
Measurement period adjustments, amortizing intangible assets	[1]	0
Amortization expense		(4,500)
Currency translation adjustments, amortizing intangible assets		(100)
Other intangible assets, net, amortizing, ending balance		68,900
Other Amortizing Intangible Assets [Member]
Other Intangible Assets [Line Items]
Other intangible assets, amortizing, gross	[2]			21,800
Accumulated amortization	[2]			(13,400)
Other Intangible Assets [Rollforward]
Other intangible assets, net, amortizing, beginning balance	[2]	8,400
Additions from new business combinations, amortizing intangible assets	[2]	300
Measurement period adjustments, amortizing intangible assets	[1],[2]	(700)
Amortization expense	[2]	(400)
Currency translation adjustments, amortizing intangible assets	[2]	0
Other intangible assets, net, amortizing, ending balance	[2]	7,600
Trade Names [Member]
Other Intangible Assets [Line Items]
Other intangible assets, non-amortizing, carrying amount		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
Pre-Qualifications [Member]
Other Intangible Assets [Line Items]
Other intangible assets, non-amortizing, carrying amount		77,600		$ 77,600
Other Intangible Assets [Rollforward]
Other intangible assets, non-amortizing, beginning balance		77,600
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		(1,200)
Other intangible assets, non-amortizing, ending balance		$ 76,400

[1] Represents adjustments to preliminary estimates of the fair values of intangible assets acquired within the measurement period for the Company’s 2017 acquisitions.

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