Goodwill and Other Intangible Assets (Rollforward of Other Intangible Assets) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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]			(300)
Amortization expense		$ (5,400)	$ (4,900)	(10,300)	$ (9,000)
Currency translation adjustments, other intangible assets				(2,200)
Other intangible assets, net, ending balance		181,515		181,515
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]			400
Amortization expense				(9,500)
Currency translation adjustments, amortizing intangible assets				(200)
Other intangible assets, net, amortizing, ending balance		64,200		64,200
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]			(800)
Currency translation adjustments, amortizing intangible assets	[2]			0
Other intangible assets, net, amortizing, ending balance	[2]	7,200		7,200
Trade Names [Member]
Other Intangible Assets [Line Items]
Other intangible assets, non-amortizing, carrying amount		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, carrying amount		75,600		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				(2,000)
Other intangible assets, non-amortizing, ending balance		$ 75,600		$ 75,600

[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.