Goodwill and Other Intangible Assets, Net - Components of Goodwill and Other Intangible Assets, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 564,300	$ 528,424	$ 487,484
Intangible assets:
Total intangible assets	191,011	185,988
Total goodwill and other intangibles assets	755,311	714,412
Accumulated amortization:
Total accumulated amortization	(118,302)	(101,206)
Goodwill and other intangible assets, net	637,009	613,206
Client Lists [Member]
Intangible assets:
Total intangible assets	181,564	177,221
Accumulated amortization:
Total accumulated amortization	(112,905)	(97,063)
Other Intangible Assets [Member]
Intangible assets:
Total intangible assets	9,447	8,767
Accumulated amortization:
Total accumulated amortization	$ (5,397)	$ (4,143)