Business Combinations - Acquisitions Accounted for Business Combinations (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	$ 721,900,000	$ 408,072,000	$ 483,933,000
Note Payable			59,000
Other Payable		552,000	25,439,000
Net Assets Acquired	756,984,000	413,715,000	530,910,000
The Wright Insurance Group, LLC. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	609,183,000
Net Assets Acquired	610,654,000
Pacific Resources Benefits Advisors, LLC (PacRes) [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	117,452,000
Axia Strategies, Inc (Axia) [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	11,694,000
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	17,184,000	12,799,000	33,543,000
The Rollins Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		16,163,000
Beecher Carlson Holdings, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid		364,256,000
Net Assets Acquired		364,256,000
ICA, Inc.[Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		20,497,000
Arrowhead General Insurance Agency Superholding Corporation [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Other Payable			22,061,000
Net Assets Acquired			422,303,000
Richard W. Endlar Insurance Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			13,423,000
Texas Security General Insurance Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			18,812,000
Behnke & Associates, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			10,339,000
Rowlands & Barranca Agency, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			11,146,000
2014 Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	721,851,000
Other Payable	1,904,000
Recorded Earn-out Payable	33,229,000
Net Assets Acquired	756,984,000
Maximum Potential Earn-out Payable	49,462,000
2014 Acquisitions [Member] | The Wright Insurance Group, LLC. [Member] | National Programs Divisions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	May 01, 2014
Cash Paid	609,183,000
Other Payable	1,471,000
Net Assets Acquired	610,654,000
2014 Acquisitions [Member] | Pacific Resources Benefits Advisors, LLC (PacRes) [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	May 01, 2014
Cash Paid	90,000,000
Recorded Earn-out Payable	27,452,000
Net Assets Acquired	117,452,000
Maximum Potential Earn-out Payable	35,000,000
2014 Acquisitions [Member] | Axia Strategies, Inc (Axia) [Member] | Wholesale Brokerage [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition	May 01, 2014
Cash Paid	9,870,000
Recorded Earn-out Payable	1,824,000
Net Assets Acquired	11,694,000
Maximum Potential Earn-out Payable	5,200,000
2014 Acquisitions [Member] | Other Acquisitions [Member] | Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	12,798,000
Other Payable	433,000
Recorded Earn-out Payable	3,953,000
Net Assets Acquired	17,184,000
Maximum Potential Earn-out Payable	9,262,000
2013 Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid		408,072,000
Other Payable		552,000
Recorded Earn-out Payable		5,091,000
Net Assets Acquired		413,715,000
Maximum Potential Earn-out Payable		16,768,000
2013 Acquisitions [Member] | Other Acquisitions [Member] | Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid		10,254,000
Other Payable		502,000
Recorded Earn-out Payable		2,043,000
Net Assets Acquired		12,799,000
Maximum Potential Earn-out Payable		7,468,000
2013 Acquisitions [Member] | The Rollins Agency, Inc. [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Jun. 01, 2013
Cash Paid		13,792,000
Other Payable		50,000
Recorded Earn-out Payable		2,321,000
Net Assets Acquired		16,163,000
Maximum Potential Earn-out Payable		4,300,000
2013 Acquisitions [Member] | Beecher Carlson Holdings, Inc. [Member] | Retail And National Programs [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Jul. 01, 2013
Cash Paid		364,256,000
Net Assets Acquired		364,256,000
2013 Acquisitions [Member] | ICA, Inc.[Member] | Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Dec. 31, 2013
Cash Paid		19,770,000
Recorded Earn-out Payable		727,000
Net Assets Acquired		20,497,000
Maximum Potential Earn-out Payable		5,000,000
2012 Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid			483,933,000
Note Payable			59,000
Other Payable			25,439,000
Recorded Earn-out Payable			21,479,000
Net Assets Acquired			530,910,000
Maximum Potential Earn-out Payable			56,170,000
2012 Acquisitions [Member] | Other Acquisitions [Member] | Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid			28,192,000
Note Payable			59,000
Other Payable			296,000
Recorded Earn-out Payable			4,996,000
Net Assets Acquired			33,543,000
Maximum Potential Earn-out Payable			14,149,000
2012 Acquisitions [Member] | Arrowhead General Insurance Agency Superholding Corporation [Member] | National Programs & Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			Jan. 09, 2012
Cash Paid			396,952,000
Other Payable			22,061,000
Recorded Earn-out Payable			3,290,000
Net Assets Acquired			422,303,000
Maximum Potential Earn-out Payable			5,000,000
2012 Acquisitions [Member] | Insurcorp & GGM Investments LLC [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			May 01, 2012
Cash Paid			15,500,000
Other Payable			900,000
Recorded Earn-out Payable			4,944,000
Net Assets Acquired			21,344,000
Maximum Potential Earn-out Payable			17,000,000
2012 Acquisitions [Member] | Richard W. Endlar Insurance Agency, Inc. [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			May 01, 2012
Cash Paid			10,825,000
Recorded Earn-out Payable			2,598,000
Net Assets Acquired			13,423,000
Maximum Potential Earn-out Payable			5,500,000
2012 Acquisitions [Member] | Texas Security General Insurance Agency, Inc. [Member] | Wholesale Brokerage [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			Sep. 01, 2012
Cash Paid			14,506,000
Other Payable			2,182,000
Recorded Earn-out Payable			2,124,000
Net Assets Acquired			18,812,000
Maximum Potential Earn-out Payable			7,200,000
2012 Acquisitions [Member] | Behnke & Associates, Inc. [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			Dec. 01, 2012
Cash Paid			9,213,000
Recorded Earn-out Payable			1,126,000
Net Assets Acquired			10,339,000
Maximum Potential Earn-out Payable			3,321,000
2012 Acquisitions [Member] | Rowlands & Barranca Agency, Inc. [Member] | Retail [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			Dec. 01, 2012
Cash Paid			8,745,000
Recorded Earn-out Payable			2,401,000
Net Assets Acquired			11,146,000
Maximum Potential Earn-out Payable			$ 4,000,000