Business Combinations - Summary of Estimated Fair Values of Net Assets Acquired (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	$ 67.5
Other current assets	953.7
Fixed assets	28.8
Noncurrent assets	14.6
Goodwill	1,223.3
Expiration lists	790.7
Non-compete agreements	12.0
Trade names	10.0
Total assets acquired	3,100.6
Current liabilities	746.6
Noncurrent liabilities	275.7
Total liabilities assumed	1,022.3
Total net assets acquired	2,078.3
Oval Group of Companies (OGC) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	23.1
Other current assets	129.6
Fixed assets	2.1
Noncurrent assets	0
Goodwill	245.7
Expiration lists	129.9
Non-compete agreements	1.0
Trade names	0.6
Total assets acquired	532.0
Current liabilities	129.5
Noncurrent liabilities	52.3
Total liabilities assumed	181.8
Total net assets acquired	350.2
Heritage Insurance Management Limited (HIM) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	2.9
Other current assets	4.9
Fixed assets	0.3
Noncurrent assets	0
Goodwill	14.8
Expiration lists	20.3
Non-compete agreements	1.3
Trade names	0
Total assets acquired	44.5
Current liabilities	6.8
Noncurrent liabilities	0
Total liabilities assumed	6.8
Total net assets acquired	37.7
MGA Insurance Group (MGA) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.2
Other current assets	8.9
Fixed assets	0.8
Noncurrent assets	0
Goodwill	27.0
Expiration lists	33.2
Non-compete agreements	0.3
Trade names	0
Total assets acquired	70.4
Current liabilities	4.1
Noncurrent liabilities	0
Total liabilities assumed	4.1
Total net assets acquired	66.3
Crombie/OAMPS (CO) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	26.6
Other current assets	695.5
Fixed assets	17.1
Noncurrent assets	7.0
Goodwill	535.9
Expiration lists	325.3
Non-compete agreements	4.2
Trade names	4.2
Total assets acquired	1,615.8
Current liabilities	490.0
Noncurrent liabilities	174.3
Total liabilities assumed	664.3
Total net assets acquired	951.5
Insurance Point, LLC (IPL) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	2.7
Other current assets	0.4
Fixed assets	0
Noncurrent assets	0
Goodwill	2.5
Expiration lists	15.3
Non-compete agreements	0.2
Trade names	0
Total assets acquired	21.1
Current liabilities	2.9
Noncurrent liabilities	0.1
Total liabilities assumed	3.0
Total net assets acquired	18.1
Trip Mate, Inc. (TMI) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0
Other current assets	7.0
Fixed assets	1.9
Noncurrent assets	0
Goodwill	5.9
Expiration lists	10.3
Non-compete agreements	0.1
Trade names	0
Total assets acquired	25.2
Current liabilities	5.8
Noncurrent liabilities	0.2
Total liabilities assumed	6.0
Total net assets acquired	19.2
Noraxis Capital Corporation (NCC) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	10.2
Other current assets	73.0
Fixed assets	4.9
Noncurrent assets	3.5
Goodwill	291.0
Expiration lists	139.7
Non-compete agreements	4.0
Trade names	4.0
Total assets acquired	530.3
Current liabilities	72.5
Noncurrent liabilities	42.6
Total liabilities assumed	115.1
Total net assets acquired	415.2
Cowles & Connell (CC) [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	0.6
Other current assets	9.1
Fixed assets	0.1
Noncurrent assets	0
Goodwill	7.8
Expiration lists	11.2
Non-compete agreements	0
Trade names	0.4
Total assets acquired	29.2
Current liabilities	9.4
Noncurrent liabilities	0
Total liabilities assumed	9.4
Total net assets acquired	19.8
Thirty-Seven Other Acquisitions [Member]
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Cash	1.2
Other current assets	25.3
Fixed assets	1.6
Noncurrent assets	4.1
Goodwill	92.7
Expiration lists	105.5
Non-compete agreements	0.9
Trade names	0.8
Total assets acquired	232.1
Current liabilities	25.6
Noncurrent liabilities	6.2
Total liabilities assumed	31.8
Total net assets acquired	$ 200.3