Business Combinations - Acquisition Method for Recording Business Combinations (Detail) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) shares
Business Acquisition [Line Items]
Common Shares Issued | shares	6,350
Common Share Value	$ 279.4
Cash Paid	249.5
Accrued Liability	7.1
Escrow Deposited	34.2
Recorded Earnout Payable	57.6
Total Recorded Purchase Price	627.8
Maximum Potential Earnout Payable	$ 202.3
e3 Financial, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	2
Common Share Value	$ 0.0
Cash Paid	9.1
Accrued Liability	0.0
Escrow Deposited	0.1
Recorded Earnout Payable	0.7
Total Recorded Purchase Price	9.9
Maximum Potential Earnout Payable	$ 7.0
Aequus Trade Credit LLC [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	11
Common Share Value	$ 0.3
Cash Paid	1.5
Accrued Liability	0.0
Escrow Deposited	0.2
Recorded Earnout Payable	1.3
Total Recorded Purchase Price	3.3
Maximum Potential Earnout Payable	$ 1.9
Cohen & Lord Insurance Brokers Limited [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	77
Common Share Value	$ 3.6
Cash Paid	2.1
Accrued Liability	0.0
Escrow Deposited	0.0
Recorded Earnout Payable	0.0
Total Recorded Purchase Price	5.7
Maximum Potential Earnout Payable	$ 0.0
Cohn Financial Group, LLC (CFG) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	407
Common Share Value	$ 19.0
Cash Paid	0.0
Accrued Liability	0.0
Escrow Deposited	0.3
Recorded Earnout Payable	4.1
Total Recorded Purchase Price	23.4
Maximum Potential Earnout Payable	$ 14.0
Excel Insurance Services, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	52
Common Share Value	$ 1.5
Cash Paid	7.3
Accrued Liability	0.0
Escrow Deposited	1.0
Recorded Earnout Payable	0.0
Total Recorded Purchase Price	9.8
Maximum Potential Earnout Payable	$ 0.0
Metcom Excess [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	49
Common Share Value	$ 1.8
Cash Paid	2.3
Accrued Liability	0.0
Escrow Deposited	0.5
Recorded Earnout Payable	0.0
Total Recorded Purchase Price	4.6
Maximum Potential Earnout Payable	$ 0.0
NationAir Aviation Insurance [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	288
Common Share Value	$ 12.3
Cash Paid	0.0
Accrued Liability	0.0
Escrow Deposited	1.3
Recorded Earnout Payable	0.0
Total Recorded Purchase Price	13.6
Maximum Potential Earnout Payable	$ 0.0
Evolution Group of Companies [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	101
Common Share Value	$ 4.7
Cash Paid	0.9
Accrued Liability	0.5
Escrow Deposited	0.4
Recorded Earnout Payable	2.9
Total Recorded Purchase Price	9.4
Maximum Potential Earnout Payable	$ 3.1
Burns-Fazzi, Brock & Associates, LLC (BFB) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	709
Common Share Value	$ 33.4
Cash Paid	0.0
Accrued Liability	0.0
Escrow Deposited	1.0
Recorded Earnout Payable	7.6
Total Recorded Purchase Price	42.0
Maximum Potential Earnout Payable	$ 27.0
Madison Risk & Insurance Services, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	232
Common Share Value	$ 10.3
Cash Paid	3.7
Accrued Liability	0.0
Escrow Deposited	1.0
Recorded Earnout Payable	1.2
Total Recorded Purchase Price	16.2
Maximum Potential Earnout Payable	$ 4.0
Integrated Healthcare Strategies, LLC (IHS) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	990
Common Share Value	$ 41.4
Cash Paid	0.5
Accrued Liability	3.1
Escrow Deposited	6.2
Recorded Earnout Payable	4.5
Total Recorded Purchase Price	55.7
Maximum Potential Earnout Payable	$ 20.8
James R. Weir Insurance Agency, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	56
Common Share Value	$ 2.5
Cash Paid	0.0
Accrued Liability	0.0
Escrow Deposited	0.3
Recorded Earnout Payable	0.8
Total Recorded Purchase Price	3.6
Maximum Potential Earnout Payable	$ 1.1
McDowall Associates Human Resources Consultants Ltd. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	34
Common Share Value	$ 1.5
Cash Paid	0.6
Accrued Liability	0.6
Escrow Deposited	0.1
Recorded Earnout Payable	0.5
Total Recorded Purchase Price	3.3
Maximum Potential Earnout Payable	$ 2.5
Vital Benefits, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	118
Common Share Value	$ 5.6
Cash Paid	0.0
Accrued Liability	0.0
Escrow Deposited	0.1
Recorded Earnout Payable	0.0
Total Recorded Purchase Price	5.7
Maximum Potential Earnout Payable	$ 0.0
Monument, LLC (ML) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	254
Common Share Value	$ 10.8
Cash Paid	4.0
Accrued Liability	0.0
Escrow Deposited	1.7
Recorded Earnout Payable	2.4
Total Recorded Purchase Price	18.9
Maximum Potential Earnout Payable	$ 5.0
Solid Benefit Guidance Limited Liability Company (SBG) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	932
Common Share Value	$ 44.0
Cash Paid	0.3
Accrued Liability	2.5
Escrow Deposited	1.0
Recorded Earnout Payable	11.8
Total Recorded Purchase Price	59.6
Maximum Potential Earnout Payable	$ 32.5
ARM Re Ltda Corredores de Reaseguros [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	0.9
Accrued Liability	0.0
Escrow Deposited	0.7
Recorded Earnout Payable	0.2
Total Recorded Purchase Price	1.8
Maximum Potential Earnout Payable	$ 0.5
National Administration Company, Inc. (NAC) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	19.1
Accrued Liability	0.0
Escrow Deposited	2.1
Recorded Earnout Payable	2.8
Total Recorded Purchase Price	24.0
Maximum Potential Earnout Payable	$ 10.8
William Gallagher Associates Insurance Brokers, Inc. (WGA) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	1,605
Common Share Value	$ 69.0
Cash Paid	75.0
Accrued Liability	0.0
Escrow Deposited	7.5
Recorded Earnout Payable	0.0
Total Recorded Purchase Price	151.5
Maximum Potential Earnout Payable	$ 0.0
North Alabama Insurance, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	188
Common Share Value	$ 7.2
Cash Paid	0.0
Accrued Liability	0.0
Escrow Deposited	0.8
Total Recorded Purchase Price	$ 8.0
Burkwald & Associates, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	3.8
Accrued Liability	0.0
Escrow Deposited	0.1
Recorded Earnout Payable	0.7
Total Recorded Purchase Price	4.6
Maximum Potential Earnout Payable	$ 3.5
Sigma II Insurance Agency [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	5.6
Accrued Liability	0.0
Escrow Deposited	0.1
Recorded Earnout Payable	1.0
Total Recorded Purchase Price	6.7
Maximum Potential Earnout Payable	$ 4.2
Christie Phoenix (Victoria) Ltd. and Discovery Insurance Services Ltd [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	15.0
Accrued Liability	0.0
Escrow Deposited	1.7
Recorded Earnout Payable	0.5
Total Recorded Purchase Price	17.2
Maximum Potential Earnout Payable	$ 2.6
Reid Manson Limited [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	6.2
Accrued Liability	0.0
Escrow Deposited	0.0
Recorded Earnout Payable	2.3
Total Recorded Purchase Price	8.5
Maximum Potential Earnout Payable	$ 4.1
Centennial Insurance Agency, LLC [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	9.5
Accrued Liability	0.0
Escrow Deposited	1.0
Recorded Earnout Payable	1.1
Total Recorded Purchase Price	11.6
Maximum Potential Earnout Payable	$ 1.4
Hawk Agency, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	4.0
Accrued Liability	0.0
Escrow Deposited	0.4
Recorded Earnout Payable	0.7
Total Recorded Purchase Price	5.1
Maximum Potential Earnout Payable	$ 1.5
Managed Healthcare Solutions, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	2.9
Accrued Liability	0.0
Escrow Deposited	0.1
Recorded Earnout Payable	1.1
Total Recorded Purchase Price	4.1
Maximum Potential Earnout Payable	$ 3.7
McPherson Benefits Group, Inc. [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	7.5
Accrued Liability	0.0
Escrow Deposited	0.1
Recorded Earnout Payable	1.5
Total Recorded Purchase Price	9.1
Maximum Potential Earnout Payable	$ 5.0
Strathearn Insurance Group Pty.Ltd (SIG) [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	0
Common Share Value	$ 0.0
Cash Paid	39.2
Accrued Liability	0.0
Escrow Deposited	2.2
Recorded Earnout Payable	0.7
Total Recorded Purchase Price	42.1
Maximum Potential Earnout Payable	$ 1.1
Fifteen Other Acquisitions [Member]
Business Acquisition [Line Items]
Common Shares Issued | shares	245
Common Share Value	$ 10.5
Cash Paid	28.5
Accrued Liability	0.4
Escrow Deposited	2.2
Recorded Earnout Payable	7.2
Total Recorded Purchase Price	48.8
Maximum Potential Earnout Payable	$ 45.0