Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail) $ in Millions	Dec. 31, 2015 USD ($)
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	$ 4.3
Other current assets	138.7
Fixed assets	8.6
Noncurrent assets	1.4
Goodwill	354.6
Expiration lists	263.0
Non-compete agreements	3.7
Trade names	0.3
Total assets acquired	774.6
Current liabilities	127.4
Noncurrent liabilities	19.4
Total liabilities assumed	146.8
Total net assets acquired	627.8
Cohn Financial Group, LLC (CFG) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.1
Goodwill	13.6
Expiration lists	9.6
Non-compete agreements	0.1
Total assets acquired	23.4
Total net assets acquired	23.4
Burns-Fazzi, Brock & Associates, LLC (BFB) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.1
Other current assets	0.3
Goodwill	17.8
Expiration lists	23.9
Non-compete agreements	0.1
Total assets acquired	42.2
Current liabilities	0.2
Total liabilities assumed	0.2
Total net assets acquired	42.0
Integrated Healthcare Strategies, LLC (IHS) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Other current assets	8.3
Fixed assets	1.5
Goodwill	31.3
Expiration lists	17.6
Non-compete agreements	0.4
Total assets acquired	59.1
Current liabilities	3.4
Total liabilities assumed	3.4
Total net assets acquired	55.7
Monument, LLC (ML) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Other current assets	0.2
Goodwill	10.1
Expiration lists	8.2
Non-compete agreements	0.2
Trade names	0.3
Total assets acquired	19.0
Current liabilities	0.1
Total liabilities assumed	0.1
Total net assets acquired	18.9
Solid Benefit Guidance Limited Liability Company (SBG) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Other current assets	1.4
Goodwill	36.5
Expiration lists	22.3
Non-compete agreements	0.1
Total assets acquired	60.3
Current liabilities	0.7
Total liabilities assumed	0.7
Total net assets acquired	59.6
National Administration Company, Inc. (NAC) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.3
Fixed assets	0.2
Goodwill	5.8
Expiration lists	17.7
Total assets acquired	24.0
Total net assets acquired	24.0
William Gallagher Associates Insurance Brokers, Inc. (WGA) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Other current assets	63.4
Fixed assets	0.6
Noncurrent assets	0.1
Goodwill	100.1
Expiration lists	49.3
Non-compete agreements	0.4
Total assets acquired	213.9
Current liabilities	62.4
Total liabilities assumed	62.4
Total net assets acquired	151.5
Strathearn Insurance Group Pty.Ltd (SIG) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Other current assets	31.3
Fixed assets	0.5
Goodwill	30.0
Expiration lists	15.9
Non-compete agreements	0.4
Total assets acquired	78.1
Current liabilities	31.5
Noncurrent liabilities	4.5
Total liabilities assumed	36.0
Total net assets acquired	42.1
Thirty Six Other Acquisitions [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	3.8
Other current assets	33.8
Fixed assets	5.8
Noncurrent assets	1.3
Goodwill	109.4
Expiration lists	98.5
Non-compete agreements	2.0
Total assets acquired	254.6
Current liabilities	29.1
Noncurrent liabilities	14.9
Total liabilities assumed	44.0
Total net assets acquired	$ 210.6