Goodwill and Intangible Assets - Goodwill activity (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Goodwill, Roll forward
Goodwill, gross, at the beginning of the year		$ 377,302
Accumulated impairment losses		(24,291)
Goodwill, net, at the beginning of the year	$ 353,011
Business combinations	25,070
Foreign currency translation adjustment	(6,558)
Goodwill, net, at the end of the year	371,523
North America
Goodwill, Roll forward
Goodwill, gross, at the beginning of the year		240,837
Goodwill, net, at the beginning of the year	240,837
Business combinations	25,030
Foreign currency translation adjustment	35
Goodwill, net, at the end of the year	265,902
Europe
Goodwill, Roll forward
Goodwill, gross, at the beginning of the year		136,465
Accumulated impairment losses		$ (24,291)
Goodwill, net, at the beginning of the year	112,174
Business combinations	40
Foreign currency translation adjustment	(6,593)
Goodwill, net, at the end of the year	$ 105,621