Goodwill and Intangible Assets - Goodwill activity (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 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	34,294
Foreign currency translation adjustment	(3,877)
Goodwill, net, at the end of the year	342,095
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	34,294
Foreign currency translation adjustment	1,579
Goodwill, net, at the end of the year	231,999
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	(5,456)
Goodwill, net, at the end of the year	$ 110,096