Goodwill and Other Intangible Assets, Net - Components of Goodwill and Other Intangible Assets, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 560,712	$ 528,424
Intangible assets:
Total intangible assets	189,688	185,988
Total goodwill and intangibles assets	750,400	714,412
Accumulated amortization:
Total accumulated amortization	(114,198)	(101,206)
Goodwill and other intangible assets, net	636,202	613,206
Client Lists [Member]
Intangible assets:
Total intangible assets	180,320	177,221
Accumulated amortization:
Total accumulated amortization	(109,105)	(97,063)
Other Intangible Assets [Member]
Intangible assets:
Total intangible assets	9,368	8,767
Accumulated amortization:
Total accumulated amortization	$ (5,093)	$ (4,143)