Business Combinations - Additions, Payments, and Net Changes, as well as Interest Expense Accretion on Estimated Acquisition Earn-Out Payables (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Document Period End Date	Dec. 31, 2018
Business Acquisition Cost- Note Payable			$ 492
Beginning balance	$ 36,175	$ 63,821	78,387
Additions to estimated acquisition earn-out payables	77,377	6,920	4,462
Payments for estimated acquisition earn-out payables	26,597	43,766	28,213
Subtotal	86,955	26,975	54,636
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(603)	(6,874)	(6,338)
Interest expense accretion	2,366	2,326	2,847
Net change in earnings from estimated acquisition earn-out payables	2,969	9,200	9,185
Ending balance	$ 89,924	$ 36,175	63,821
Services | Social Security Advocates for the Disabled LLC [Member]
Business Acquisition [Line Items]
Business Acquisition Cost- Note Payable			492
Wholesale Brokerage [Member] | Morstan General Agency, Inc [Member]
Business Acquisition [Line Items]
Business Acquisition Cost- Note Payable			0
Various [Member] | Other Acquisitions [Member]
Business Acquisition [Line Items]
Business Acquisition Cost- Note Payable			$ 0