Goodwill and Other Intangible Assets, Net - Changes in Carrying Amount of Goodwill by Operating Segment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 487,484	$ 447,685
Additions	40,940	39,799
Goodwill	528,424	487,484
Financial Services [Member]
Finite-Lived Intangible Assets [Line Items]
Goodwill	271,330	267,485
Additions	35,531	3,845
Goodwill	306,861	271,330
Benefits and Insurance Services [Member]
Finite-Lived Intangible Assets [Line Items]
Goodwill	214,488	178,534
Additions	5,409	35,954
Goodwill	219,897	214,488
National Practices [Member]
Finite-Lived Intangible Assets [Line Items]
Goodwill	1,666	1,666
Goodwill	$ 1,666	$ 1,666