Goodwill and Other Intangible Assets, Net - Components of Goodwill and Other Intangible Assets, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 551,367	$ 528,424
Intangible assets:
Total intangible assets	186,014	185,988
Total goodwill and intangibles assets	737,381	714,412
Accumulated amortization:
Total accumulated amortization	(105,425)	(101,206)
Goodwill and other intangible assets, net	631,956	613,206
Client Lists [Member]
Intangible assets:
Total intangible assets	176,810	177,221
Accumulated amortization:
Total accumulated amortization	(100,990)	(97,063)
Other Intangible Assets [Member]
Intangible assets:
Total intangible assets	9,204	8,767
Accumulated amortization:
Total accumulated amortization	$ (4,435)	$ (4,143)