Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Fair Value Of Assets And Liabilities Statement [Line Items]
Payments to Acquire Businesses, Gross	$ 26,478	$ 115,300
Recorded Earn-Out Payable	1,332
Other current assets	98
Fixed assets	47
Goodwill	27,580
Purchased customer accounts	12,858
Non-compete agreements	595
Total assets acquired	41,178
Other current liabilities	(1,284)
Deferred Tax Liabilities, Net	(689)
Total liabilities assumed	(1,973)
Net assets acquired	39,205
Maximum Potential Earn- Out Payable	11,605
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	98
Fixed assets	47
Goodwill	27,580
Purchased customer accounts	12,858
Non-compete agreements	595
Total assets acquired	41,178
Other current liabilities	(1,284)
Deferred Tax Liabilities, Net	(689)
Total liabilities assumed	(1,973)
Net assets acquired	39,205
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 771