Goodwill and Other Intangible Assets, Net - Components of Goodwill and Other Intangible Assets, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 566,009	$ 564,300
Intangible assets:
Total intangible assets	191,634	191,011
Total goodwill and intangibles assets	757,643	755,311
Accumulated amortization:
Total accumulated amortization	(121,762)	(118,302)
Goodwill and other intangible assets, net	635,881	637,009
Client Lists [Member]
Intangible assets:
Total intangible assets	182,184	181,564
Accumulated amortization:
Total accumulated amortization	(116,103)	(112,905)
Other Intangible Assets [Member]
Intangible assets:
Total intangible assets	9,450	9,447
Accumulated amortization:
Total accumulated amortization	$ (5,659)	$ (5,397)