Goodwill and Other Intangible Assets, Net - Components of Goodwill and Other Intangible Assets, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 683,083	$ 588,213	$ 564,300
Intangibles :
Total intangibles	218,328	198,780
Total goodwill and other intangibles assets	901,411	786,993
Accumulated amortization:
Total accumulated amortization	(144,661)	(132,322)
Goodwill and other intangible assets, net	756,750	654,671
Client lists
Intangibles :
Total intangibles	207,084	188,898
Accumulated amortization:
Total accumulated amortization	(137,284)	(125,887)
Other intangibles
Intangibles :
Total intangibles	11,244	9,882
Accumulated amortization:
Total accumulated amortization	$ (7,377)	$ (6,435)