Intangible Assets - Goodwill and Net Carrying Amounts (Details) - USD ($) $ in Thousands		12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Goodwill
Goodwill gross, beginning balance		$ 583,078	$ 598,939
Accumulated impairment loss, beginning balance		(50,003)	(50,003)
Goodwill net, beginning balance		533,075	548,936
Acquisitions		348,464
Foreign currency translation adjustments		33,438	(15,861)
Impairment loss		(140,038)
Goodwill gross, ending balance		964,980	583,078
Accumulated impairment loss, ending balance		(190,041)	(50,003)
Goodwill net, ending balance		774,939	533,075
Trade Name
Intangible Assets
Indefinite lived intangible asset, Beginning balance		619	2,316
Reclassifications to definite-lived			(1,700)
Indefinite lived intangible asset, foreign currency translation adjustments		40	3
Indefinite lived intangible asset, Ending balance		659	619
North America
Goodwill
Goodwill gross, beginning balance		452,232	452,270
Goodwill net, beginning balance		452,232	452,270
Acquisitions		107,442
Foreign currency translation adjustments		6,043	(38)
Goodwill gross, ending balance		565,717	452,232
Goodwill net, ending balance		565,717	452,232
North America | Trade Name
Intangible Assets
Indefinite lived intangible asset, Beginning balance		200	1,900
Reclassifications to definite-lived			(1,700)
Indefinite lived intangible asset, Ending balance		200	200
Europe And Africa
Goodwill
Goodwill gross, beginning balance		130,846	146,669
Accumulated impairment loss, beginning balance		(50,003)	(50,003)
Goodwill net, beginning balance		80,843	96,666
Acquisitions		99,465
Foreign currency translation adjustments		16,238	(15,823)
Goodwill gross, ending balance		246,549	130,846
Accumulated impairment loss, ending balance		(50,003)	(50,003)
Goodwill net, ending balance		196,546	80,843
Europe And Africa | Trade Name
Intangible Assets
Indefinite lived intangible asset, Beginning balance		419	416
Indefinite lived intangible asset, foreign currency translation adjustments		40	3
Indefinite lived intangible asset, Ending balance		459	$ 419
Australia And New Zealand
Goodwill
Acquisitions		141,557
Foreign currency translation adjustments		11,157
Impairment loss	$ (140,000)	(140,038)
Goodwill gross, ending balance		152,714
Accumulated impairment loss, ending balance		(140,038)
Goodwill net, ending balance		$ 12,676