Business Combinations - Acquisition Method for Recording Business Combinations (Detail) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Business Acquisition [Line Items]
Common Shares Issued	5,739
Common Share Value	$ 252.1
Cash Paid	1,914.5
Accrued Liability	0
Escrow Deposited	42.4
Recorded Earnout Payable	80.5
Total Recorded Purchase Price	2,289.5
Maximum Potential Earnout Payable	198.8
Benefit Development Group, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued	46
Common Share Value	2.0
Cash Paid	0.7
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	0.6
Total Recorded Purchase Price	3.4
Maximum Potential Earnout Payable	2.0
Kent, Kent & Tingle [Member]
Business Acquisition [Line Items]
Common Shares Issued	229
Common Share Value	9.1
Cash Paid	3.5
Accrued Liability	0
Escrow Deposited	1.4
Recorded Earnout Payable	3.5
Total Recorded Purchase Price	17.5
Maximum Potential Earnout Payable	7.8
L&R Benefits, LLC [Member]
Business Acquisition [Line Items]
Common Shares Issued	115
Common Share Value	5.3
Cash Paid	1.8
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	1.0
Total Recorded Purchase Price	8.2
Maximum Potential Earnout Payable	6.0
Spataro Insurance Agency, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued	47
Common Share Value	2.0
Cash Paid	0
Accrued Liability	0
Escrow Deposited	0.2
Recorded Earnout Payable	0.4
Total Recorded Purchase Price	2.6
Maximum Potential Earnout Payable	0.6
Tudor Risk Services, LLC [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	2.1
Accrued Liability	0
Escrow Deposited	0.2
Recorded Earnout Payable	0.1
Total Recorded Purchase Price	2.4
Maximum Potential Earnout Payable	1.3
American Wholesalers Underwriting Ltd [Member]
Business Acquisition [Line Items]
Common Shares Issued	133
Common Share Value	6.0
Cash Paid	5.7
Accrued Liability	0
Escrow Deposited	0.5
Recorded Earnout Payable	0
Total Recorded Purchase Price	12.2
Maximum Potential Earnout Payable	0
Mike Henry Insurance Brokers Limited [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	9.6
Accrued Liability	0
Escrow Deposited	1.7
Recorded Earnout Payable	4.2
Total Recorded Purchase Price	15.5
Maximum Potential Earnout Payable	5.0
Oval Group of Companies (OGC) [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	338.4
Accrued Liability	0
Escrow Deposited	11.8
Recorded Earnout Payable	0
Total Recorded Purchase Price	350.2
Maximum Potential Earnout Payable	0
Heritage Insurance Management Limited (HIM) [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	33.9
Accrued Liability	0
Escrow Deposited	3.8
Recorded Earnout Payable	0
Total Recorded Purchase Price	37.7
Maximum Potential Earnout Payable	0
MGA Insurance Group (MGA) [Member]
Business Acquisition [Line Items]
Common Shares Issued	547
Common Share Value	25.1
Cash Paid	26.8
Accrued Liability	0
Escrow Deposited	2.5
Recorded Earnout Payable	11.9
Total Recorded Purchase Price	66.3
Maximum Potential Earnout Payable	20.0
Shilling Limited [Member]
Business Acquisition [Line Items]
Common Shares Issued	198
Common Share Value	8.9
Cash Paid	1.7
Accrued Liability	0
Escrow Deposited	1.2
Recorded Earnout Payable	1.0
Total Recorded Purchase Price	12.8
Maximum Potential Earnout Payable	8.4
Sunderland Insurance Services, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued	204
Common Share Value	9.2
Cash Paid	2.4
Accrued Liability	0
Escrow Deposited	0.6
Recorded Earnout Payable	0
Total Recorded Purchase Price	12.2
Maximum Potential Earnout Payable	0
Plus Companies, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued	221
Common Share Value	9.3
Cash Paid	3.4
Accrued Liability	0
Escrow Deposited	0.8
Recorded Earnout Payable	0
Total Recorded Purchase Price	13.5
Maximum Potential Earnout Payable	0
Tri-State General Insurance Agency, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued	47
Common Share Value	2.2
Cash Paid	0.6
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	0
Total Recorded Purchase Price	2.9
Maximum Potential Earnout Payable	0
Crombie/OAMPS (CO) [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	993.1
Accrued Liability	0
Escrow Deposited	0
Recorded Earnout Payable	0
Total Recorded Purchase Price	993.1
Maximum Potential Earnout Payable	0
Foundation Strategies, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued	46
Common Share Value	2.0
Cash Paid	0.7
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	0.4
Total Recorded Purchase Price	3.2
Maximum Potential Earnout Payable	3.0
Insurance Point, LLC (IPL) [Member]
Business Acquisition [Line Items]
Common Shares Issued	255
Common Share Value	11.2
Cash Paid	3.9
Accrued Liability	0
Escrow Deposited	0.5
Recorded Earnout Payable	2.6
Total Recorded Purchase Price	18.2
Maximum Potential Earnout Payable	24.4
Trip Mate, Inc. (TMI) [Member]
Business Acquisition [Line Items]
Common Shares Issued	209
Common Share Value	8.4
Cash Paid	3.9
Accrued Liability	0
Escrow Deposited	1.0
Recorded Earnout Payable	3.7
Total Recorded Purchase Price	17.0
Maximum Potential Earnout Payable	12.5
Noraxis Capital Corporation (NCC) [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	413.3
Accrued Liability	0
Escrow Deposited	1.9
Recorded Earnout Payable	0
Total Recorded Purchase Price	415.2
Maximum Potential Earnout Payable	0
Cowles & Connell (CC) [Member]
Business Acquisition [Line Items]
Common Shares Issued	331
Common Share Value	14.8
Cash Paid	4.2
Accrued Liability	0
Escrow Deposited	0.8
Recorded Earnout Payable	0
Total Recorded Purchase Price	19.8
Maximum Potential Earnout Payable	0
Denman Consulting Services [Member]
Business Acquisition [Line Items]
Common Shares Issued	40
Common Share Value	1.7
Cash Paid	0.6
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	0.3
Total Recorded Purchase Price	2.7
Maximum Potential Earnout Payable	1.6
Minvielle & Chastanet Insurance Brokers [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	5.0
Accrued Liability	0
Escrow Deposited	0
Recorded Earnout Payable	3.0
Total Recorded Purchase Price	8.0
Maximum Potential Earnout Payable	5.0
Baker Tilly Financial Management Limited [Member]
Business Acquisition [Line Items]
Common Shares Issued	185
Common Share Value	8.7
Cash Paid	2.3
Accrued Liability	0
Escrow Deposited	0.7
Recorded Earnout Payable	4.7
Total Recorded Purchase Price	16.4
Maximum Potential Earnout Payable	5.4
Benfield Group [Member]
Business Acquisition [Line Items]
Common Shares Issued	82
Common Share Value	3.8
Cash Paid	1.1
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	0.9
Total Recorded Purchase Price	5.9
Maximum Potential Earnout Payable	3.5
Everett James, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued	52
Common Share Value	2.4
Cash Paid	0.7
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	0.8
Total Recorded Purchase Price	4.0
Maximum Potential Earnout Payable	4.0
Hagedorn & Company [Member]
Business Acquisition [Line Items]
Common Shares Issued	281
Common Share Value	11.5
Cash Paid	0
Accrued Liability	0
Escrow Deposited	1.3
Recorded Earnout Payable	0
Total Recorded Purchase Price	12.8
Maximum Potential Earnout Payable	0
Parmia Pty Ltd. [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	1.7
Accrued Liability	0
Escrow Deposited	0
Recorded Earnout Payable	1.2
Total Recorded Purchase Price	2.9
Maximum Potential Earnout Payable	1.2
Bennett and Shade Company [Member]
Business Acquisition [Line Items]
Common Shares Issued	35
Common Share Value	1.5
Cash Paid	0.5
Accrued Liability	0
Escrow Deposited	0.2
Recorded Earnout Payable	0
Total Recorded Purchase Price	2.2
Maximum Potential Earnout Payable	0
Insurance Associates Inc [Member]
Business Acquisition [Line Items]
Common Shares Issued	169
Common Share Value	7.2
Cash Paid	0
Accrued Liability	0
Escrow Deposited	0.8
Recorded Earnout Payable	1.1
Total Recorded Purchase Price	9.1
Maximum Potential Earnout Payable	3.0
Forker Company [Member]
Business Acquisition [Line Items]
Common Shares Issued	24
Common Share Value	1.1
Cash Paid	0.3
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	1.6
Total Recorded Purchase Price	3.1
Maximum Potential Earnout Payable	2.2
Discovery Benefit Solutions Inc [Member]
Business Acquisition [Line Items]
Common Shares Issued	115
Common Share Value	5.4
Cash Paid	0
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	1.4
Total Recorded Purchase Price	6.9
Maximum Potential Earnout Payable	4.5
Miller Harrison Insurance Services [Member]
Business Acquisition [Line Items]
Common Shares Issued	38
Common Share Value	1.8
Cash Paid	0
Accrued Liability	0
Escrow Deposited	0
Recorded Earnout Payable	0.6
Total Recorded Purchase Price	2.4
Maximum Potential Earnout Payable	1.0
SGB-NIA Insurance Brokers (SGB) [Member]
Business Acquisition [Line Items]
Common Shares Issued	449
Common Share Value	18.7
Cash Paid	7.2
Accrued Liability	0
Escrow Deposited	2.9
Recorded Earnout Payable	4.2
Total Recorded Purchase Price	33.0
Maximum Potential Earnout Payable	5.2
Titan Group LLC [Member]
Business Acquisition [Line Items]
Common Shares Issued	49
Common Share Value	2.4
Cash Paid	0
Accrued Liability	0
Escrow Deposited	0.1
Recorded Earnout Payable	0.4
Total Recorded Purchase Price	2.9
Maximum Potential Earnout Payable	2.0
Instrat Insurance Brokers [Member]
Business Acquisition [Line Items]
Common Shares Issued	0
Common Share Value	0
Cash Paid	16.2
Accrued Liability	0
Escrow Deposited	0
Recorded Earnout Payable	9.5
Total Recorded Purchase Price	25.7
Maximum Potential Earnout Payable	9.5
O Gorman & Young Incorporated (OGY) [Member]
Business Acquisition [Line Items]
Common Shares Issued	554
Common Share Value	23.8
Cash Paid	0
Accrued Liability	0
Escrow Deposited	2.7
Recorded Earnout Payable	6.4
Total Recorded Purchase Price	32.9
Maximum Potential Earnout Payable	12.5
Independent Benefit Services, Inc. (IBS) [Member]
Business Acquisition [Line Items]
Common Shares Issued	395
Common Share Value	17.8
Cash Paid	6.2
Accrued Liability	0
Escrow Deposited	0.8
Recorded Earnout Payable	1.3
Total Recorded Purchase Price	26.1
Maximum Potential Earnout Payable	14.3
Affinity Marketing Group [Member]
Business Acquisition [Line Items]
Common Shares Issued	72
Common Share Value	3.1
Cash Paid	0
Accrued Liability	0
Escrow Deposited	0.3
Recorded Earnout Payable	2.2
Total Recorded Purchase Price	5.6
Maximum Potential Earnout Payable	5.2
Blue Holdings Group of Companies [Member]
Business Acquisition [Line Items]
Common Shares Issued	227
Common Share Value	10.9
Cash Paid	4.5
Accrued Liability	0
Escrow Deposited	1.7
Recorded Earnout Payable	1.2
Total Recorded Purchase Price	18.3
Maximum Potential Earnout Payable	6.6
Twenty One Other Acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued	344
Common Share Value	14.8
Cash Paid	18.5
Accrued Liability	0
Escrow Deposited	1.1
Recorded Earnout Payable	10.3
Total Recorded Purchase Price	44.7
Maximum Potential Earnout Payable	$ 21.1