Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Fair Value Of Assets And Liabilities Statement [Line Items]
Payments to Acquire Businesses, Gross	$ 11,526	$ 109,400
Recorded Earn-Out Payable	282
Other current assets	93
Fixed assets	35
Goodwill	16,702
Purchased customer accounts	7,279
Non-compete agreements	552
Total assets acquired	24,661
Other current liabilities	(1,095)
Deferred Tax Liabilities, Net	(689)
Total liabilities assumed	(1,784)
Net assets acquired	22,877
Maximum Potential Earn- Out Payable	3,305
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	93
Fixed assets	35
Goodwill	16,702
Purchased customer accounts	7,279
Non-compete agreements	552
Total assets acquired	24,661
Other current liabilities	(1,095)
Deferred Tax Liabilities, Net	(689)
Total liabilities assumed	(1,784)
Net assets acquired	22,877
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 878