Goodwill and Other Intangible Assets, Net - Components of Goodwill and Other Intangible Assets, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 478,551	$ 447,685
Intangible assets:
Total intangible assets	176,336	154,683
Total goodwill and intangibles assets	654,887	602,368
Accumulated amortization:
Total accumulated amortization	(79,036)	(66,715)
Goodwill and other intangible assets, net	575,851	535,653
Client Lists [Member]
Intangible assets:
Total intangible assets	168,471	147,706
Accumulated amortization:
Total accumulated amortization	(76,497)	(65,037)
Other Intangible Assets [Member]
Intangible assets:
Total intangible assets	7,865	6,977
Accumulated amortization:
Total accumulated amortization	$ (2,539)	$ (1,678)