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, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Business Acquisition Cost- Note Payable		$ 492
Beginning balance	$ 63,821	78,387	$ 75,283
Additions to estimated acquisition earn-out payables	6,920	4,462	36,899
Payments for estimated acquisition earn-out payables	43,766	28,213	36,798
Subtotal	26,975	54,636	75,384
Net change in earnings from estimated acquisition earn-out payables:
Change in fair value on estimated acquisition earn-out payables	(6,874)	(6,338)	(13)
Interest expense accretion	2,326	2,847	2,990
Net change in earnings from estimated acquisition earn-out payables	9,200	9,185	3,003
Ending balance	$ 36,175	63,821	$ 78,387
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