Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Fair Value Of Assets And Liabilities Statement [Line Items]
Payments to Acquire Businesses, Gross	$ 105,056	$ 720,100
Recorded Earn-Out Payable	27,134
Other current assets	2,656
Fixed assets	192
Goodwill	101,509
Purchased customer accounts	31,166
Non-compete agreements	164
Total assets acquired	135,687
Other current liabilities	(3,760)
Deferred Tax Liabilities, Net	2,576
Other liabilities	(1,408)
Total liabilities assumed	(2,592)
Net assets acquired	133,095
Maximum Potential Earn- Out Payable	49,704
Liberty Insurance Brokers, Inc. and Affiliates (Liberty)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	2,486
Fixed assets	40
Goodwill	8,682
Purchased customer accounts	4,289
Non-compete agreements	24
Total assets acquired	15,521
Other current liabilities	(42)
Deferred Tax Liabilities, Net	0
Other liabilities	(2,043)
Total liabilities assumed	(2,085)
Net assets acquired	13,436
Spain Agency, Inc.
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0
Fixed assets	50
Goodwill	16,169
Purchased customer accounts	7,430
Non-compete agreements	21
Total assets acquired	23,670
Other current liabilities	(214)
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	(214)
Net assets acquired	23,456
Bellingham Underwriters, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets	0
Business Acquisitions Purchase Price Allocation Property Plant And Equipment	25
Goodwill	9,608
Purchased customer accounts	3,223
Non-compete agreements	21
Business Acquisitions Purchase Price Allocation Assets Acquired	12,877
Other current liabilities	(48)
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	(48)
Net assets acquired	12,829
Fitness Insurance, LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets	0
Business Acquisitions Purchase Price Allocation Property Plant And Equipment	17
Goodwill	8,084
Purchased customer accounts	3,740
Non-compete agreements	0
Total assets acquired	11,841
Other current liabilities	0
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	0
Net assets acquired	11,841
Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	0
Fixed assets	41
Goodwill	46,677
Purchased customer accounts	17,702
Non-compete agreements	21
Total assets acquired	64,441
Other current liabilities	0
Deferred Tax Liabilities, Net	0
Other liabilities	0
Total liabilities assumed	0
Net assets acquired	64,441
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	170
Fixed assets	19
Goodwill	12,289
Purchased customer accounts	(5,218)
Non-compete agreements	77
Total assets acquired	7,337
Other current liabilities	(3,456)
Deferred Tax Liabilities, Net	2,576
Other liabilities	635
Total liabilities assumed	(245)
Net assets acquired	7,092
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 2,967
Wholesale Brokerage | Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition	Jun. 01, 2015