Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	$ 2,094,000
Payments to Acquire Businesses, Gross	115,313,000	$ 109,900,000
Recorded Earn-Out Payable	3,828,000
Other current assets	4,277,000
Fixed assets	654,000
Goodwill	85,682,000
Purchased customer accounts	46,751,000
Non-compete agreements	195,000
Total assets acquired	139,653,000
Other current liabilities	(6,285,000)
Deferred Tax Liabilities, Net	(3,230,000)
Total liabilities assumed	(9,515,000)
Net assets acquired	130,138,000
Maximum Potential Earn- Out Payable	13,722,000
SSAD [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	2,094,000
Other current assets	1,042,000
Fixed assets	307,000
Goodwill	22,352,000
Purchased customer accounts	13,069,000
Non-compete agreements	72,000
Total assets acquired	38,936,000
Other current liabilities	(1,717,000)
Deferred Tax Liabilities, Net	(3,230,000)
Total liabilities assumed	(4,947,000)
Net assets acquired	33,989,000
Morstan General Agency, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	2,482,000
Fixed assets	300,000
Goodwill	51,454,000
Purchased customer accounts	26,481,000
Non-compete agreements	39,000
Total assets acquired	80,756,000
Other current liabilities	(1,415,000)
Deferred Tax Liabilities, Net	0
Total liabilities assumed	(1,415,000)
Net assets acquired	79,341,000
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Other current assets	753,000
Fixed assets	47,000
Goodwill	11,876,000
Purchased customer accounts	7,201,000
Non-compete agreements	84,000
Total assets acquired	19,961,000
Other current liabilities	(3,153,000)
Deferred Tax Liabilities, Net	0
Total liabilities assumed	(3,153,000)
Net assets acquired	16,808,000
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 51,477,000