Acquisitions Accounted for Business Combinations (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	$ 379,694	$ 443,475
Note Payable		59
Net Assets Acquired	381,407	484,365
Arrowhead General Insurance Agency Superholding Corporation [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	(454)	423,859
Insurcorp & GGM Investments LLC [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	(834)	21,344
Richard W. Endlar Insurance Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	220
Texas Security General Insurance Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	(107)	18,812
The Rollins Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	16,163
Beecher Carlson Holdings, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	364,643
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	1,776	20,350
2013 Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	379,712
Other Payable	(369)
Recorded Earn-out Payable	2,064
Net Assets Acquired	381,407
Maximum Potential Earn-out Payable	5,048
2013 Acquisitions [Member] | Arrowhead General Insurance Agency Superholding Corporation [Member] | National Programs & Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition	Jan. 09, 2012
Other Payable	(454)
Net Assets Acquired	(454)
2013 Acquisitions [Member] | Insurcorp & GGM Investments LLC [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition	May 01, 2012
Recorded Earn-out Payable	(834)
Net Assets Acquired	(834)
2013 Acquisitions [Member] | Richard W. Endlar Insurance Agency, Inc. [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition	May 01, 2012
Recorded Earn-out Payable	220
Net Assets Acquired	220
2013 Acquisitions [Member] | Texas Security General Insurance Agency, Inc. [Member] | Wholesale Brokerage [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition	Sep. 01, 2012
Recorded Earn-out Payable	(107)
Net Assets Acquired	(107)
2013 Acquisitions [Member] | The Rollins Agency, Inc. [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition	Jun. 01, 2013
Cash Paid	13,792
Other Payable	50
Recorded Earn-out Payable	2,321
Net Assets Acquired	16,163
Maximum Potential Earn-out Payable	4,300
2013 Acquisitions [Member] | Beecher Carlson Holdings, Inc. [Member] | Retail And National Programs [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition	Jul. 01, 2013
Cash Paid	364,643
Net Assets Acquired	364,643
2013 Acquisitions [Member] | Other Acquisitions [Member] | Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	1,277
Other Payable	35
Recorded Earn-out Payable	464
Net Assets Acquired	1,776
Maximum Potential Earn-out Payable	748
2012 Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid		443,475
Note Payable		59
Other Payable		25,776
Recorded Earn-out Payable		15,055
Net Assets Acquired		484,365
Maximum Potential Earn-out Payable		39,435
2012 Acquisitions [Member] | Arrowhead General Insurance Agency Superholding Corporation [Member] | National Programs & Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition		Jan. 09, 2012
Cash Paid		397,531
Other Payable		22,694
Recorded Earn-out Payable		3,634
Net Assets Acquired		423,859
Maximum Potential Earn-out Payable		5,000
2012 Acquisitions [Member] | Insurcorp & GGM Investments LLC [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition		May 01, 2012
Cash Paid		15,500
Other Payable		900
Recorded Earn-out Payable		4,944
Net Assets Acquired		21,344
Maximum Potential Earn-out Payable		17,000
2012 Acquisitions [Member] | Texas Security General Insurance Agency, Inc. [Member] | Wholesale Brokerage [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Date of Acquisition		Sep. 01, 2012
Cash Paid		14,506
Other Payable		2,182
Recorded Earn-out Payable		2,124
Net Assets Acquired		18,812
Maximum Potential Earn-out Payable		7,200
2012 Acquisitions [Member] | Other Acquisitions [Member] | Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid		15,938
Note Payable		59
Recorded Earn-out Payable		4,353
Net Assets Acquired		20,350
Maximum Potential Earn-out Payable		$ 10,235