Goodwill - Rollforward (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Goodwill
Beginning balance	$ 585,942,000	$ 549,063,000
Acquisitions - preliminary allocation	26,553,000	64,950,000
Acquisition remeasurements	(2,783,000)	(3,663,000)
Sale of business unit	(1,343,000)
Currency translation	(5,869,000)	(24,408,000)
Ending balance	602,500,000	585,942,000	$ 549,063,000
Accumulated impairment loss	$ 1,600,000	1,600,000	1,600,000
Number of reporting units | segment	3
Percentage of fair value in excess of carrying amount	33.00%
Impairment loss	$ 0	0	0
North America
Goodwill
Beginning balance	223,562,000	201,560,000
Acquisitions - preliminary allocation	26,553,000	17,645,000
Acquisition remeasurements	(1,535,000)	4,881,000
Sale of business unit	(1,343,000)
Currency translation	265,000	(524,000)
Ending balance	247,502,000	223,562,000	201,560,000
Europe
Goodwill
Beginning balance	279,688,000	268,162,000
Acquisitions - preliminary allocation	0	30,167,000
Acquisition remeasurements	0	(3,317,000)
Sale of business unit	0
Currency translation	(5,776,000)	(15,324,000)
Ending balance	273,912,000	279,688,000	268,162,000
Australasia
Goodwill
Beginning balance	82,692,000	79,341,000
Acquisitions - preliminary allocation	0	17,138,000
Acquisition remeasurements	(1,248,000)	(5,227,000)
Sale of business unit	0
Currency translation	(358,000)	(8,560,000)
Ending balance	$ 81,086,000	$ 82,692,000	$ 79,341,000