Goodwill and Other Intangible Assets, Net - Components of Goodwill and Other Intangible Assets, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 588,213	$ 564,300
Intangible assets:
Total intangible assets	198,998	191,011
Total goodwill and intangibles assets	787,211	755,311
Accumulated amortization:
Total accumulated amortization	(128,769)	(118,302)
Goodwill and other intangible assets, net	658,442	637,009
Client Lists [Member]
Intangible assets:
Total intangible assets	189,120	181,564
Accumulated amortization:
Total accumulated amortization	(122,595)	(112,905)
Other Intangible Assets [Member]
Intangible assets:
Total intangible assets	9,878	9,447
Accumulated amortization:
Total accumulated amortization	$ (6,174)	$ (5,397)