Goodwill and Intangible Assets - Goodwill activity (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Goodwill, Roll forward
Goodwill, gross, at the beginning of the year			$ 335,969
Accumulated impairment losses			(24,291)
Goodwill, net, at the beginning of the year	$ 311,678
Business combinations	10,986
Foreign currency translation adjustment	(6,459)
Goodwill, net, at the end of the year	316,205
Goodwill
Goodwill impairment	0	$ 0
Long-lived asset impairment	0	$ 0
North America
Goodwill, Roll forward
Goodwill, gross, at the beginning of the year			196,126
Goodwill, net, at the beginning of the year	196,126
Business combinations	10,986
Foreign currency translation adjustment	(307)
Goodwill, net, at the end of the year	206,805
Europe
Goodwill, Roll forward
Goodwill, gross, at the beginning of the year			139,843
Accumulated impairment losses			$ (24,291)
Goodwill, net, at the beginning of the year	115,552
Foreign currency translation adjustment	(6,152)
Goodwill, net, at the end of the year	$ 109,400