Business Combinations - Acquisitions Accounted for Business Combinations (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	$ 109,926	$ 720,100
Other Payable	995
Recorded Earn-Out Payable	29,496
Net Assets Acquired	140,417
Maximum Potential Earn- Out Payable	53,079
Liberty Insurance Brokers, Inc. and Affiliates (Liberty)
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	$ 14,981
Liberty Insurance Brokers, Inc. and Affiliates (Liberty) | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	Feb. 01, 2015
Cash Paid	$ 12,000
Other Payable	0
Recorded Earn-Out Payable	2,981
Net Assets Acquired	14,981
Maximum Potential Earn- Out Payable	3,750
Spain Agency, Inc.
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	$ 23,323
Spain Agency, Inc. | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	Mar. 01, 2015
Cash Paid	$ 20,706
Other Payable	0
Recorded Earn-Out Payable	2,617
Net Assets Acquired	23,323
Maximum Potential Earn- Out Payable	9,162
Bellingham Underwriters, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	$ 12,829
Bellingham Underwriters, Inc. [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	May 01, 2015
Bellingham Underwriters, Inc. [Member] | National Programs Divisions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	$ 9,007
Other Payable	500
Recorded Earn-Out Payable	3,322
Net Assets Acquired	12,829
Maximum Potential Earn- Out Payable	4,400
Fitness Insurance, LLC [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	$ 11,841
Fitness Insurance, LLC [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	Jun. 01, 2015
Cash Paid	$ 9,455
Other Payable	0
Recorded Earn-Out Payable	2,386
Net Assets Acquired	11,841
Maximum Potential Earn- Out Payable	3,500
Other
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	64,441
Other | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	49,600
Other Payable	400
Recorded Earn-Out Payable	14,441
Net Assets Acquired	64,441
Maximum Potential Earn- Out Payable	$ 26,000
Other | Wholesale Brokerage
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	Jun. 01, 2015
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	$ 13,002
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	9,158
Other Payable	95
Recorded Earn-Out Payable	3,749
Net Assets Acquired	13,002
Maximum Potential Earn- Out Payable	$ 6,267