Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Fair Value Of Assets And Liabilities Statement [Line Items]
Payments to Acquire Businesses, Gross	$ 109,926,000	$ 720,100,000
Recorded Earn-Out Payable	29,496,000
Other current assets	2,656,000
Fixed assets	205,000
Goodwill	100,632,000
Purchased customer accounts	39,294,000
Non-compete agreements	210,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	14,000
Total assets acquired	143,011,000
Other current liabilities	(3,762,000)
Deferred Tax Liabilities, Net	2,576,000
Other liabilities	(1,408,000)
Total liabilities assumed	(2,594,000)
Net assets acquired	140,417,000
Maximum Potential Earn- Out Payable	53,079,000
Liberty Insurance Brokers, Inc. and Affiliates (Liberty)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	2,486,000
Fixed assets	40,000
Goodwill	10,010,000
Purchased customer accounts	4,506,000
Non-compete agreements	24,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	17,066,000
Other current liabilities	(42,000)
Deferred Tax Liabilities, Net	0
Other liabilities	(2,043,000)
Total liabilities assumed	(2,085,000)
Net assets acquired	14,981,000
Spain Agency, Inc.
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0
Fixed assets	50,000
Goodwill	16,036,000
Purchased customer accounts	7,430,000
Non-compete agreements	21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	23,537,000
Other current liabilities	(214,000)
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	(214,000)
Net assets acquired	23,323,000
Bellingham Underwriters, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0
Fixed assets	25,000
Goodwill	9,608,000
Purchased customer accounts	3,223,000
Non-compete agreements	21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	12,877,000
Other current liabilities	(48,000)
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	(48,000)
Net assets acquired	12,829,000
Fitness Insurance, LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0
Fixed assets	17,000
Goodwill	8,084,000
Purchased customer accounts	3,740,000
Non-compete agreements	0
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	11,841,000
Other current liabilities	0
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	0
Net assets acquired	11,841,000
Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0
Fixed assets	41,000
Goodwill	39,865,000
Purchased customer accounts	24,500,000
Non-compete agreements	21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	14,000
Total assets acquired	64,441,000
Other current liabilities	0
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	0
Net assets acquired	64,441,000
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	170,000
Fixed assets	32,000
Goodwill	17,029,000
Purchased customer accounts	(4,105,000)
Non-compete agreements	123,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Total assets acquired	13,249,000
Other current liabilities	(3,458,000)
Deferred Tax Liabilities, Net	2,576,000
Other liabilities	635,000
Total liabilities assumed	(247,000)
Net assets acquired	13,002,000
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 4,605,000
Wholesale Brokerage | Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition	Jun. 01, 2015