Segment Information - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($) Entity	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting Information [Line Items]
Net premiums earned	$ 19,899.1	$ 18,398.5	$ 17,103.4
Depreciation	$ 103.7	97.1	101.3
Underwriting Operations \| Personal Lines \| Agency Channel \| Lower Limit
Segment Reporting Information [Line Items]
Independent insurance agencies and brokerages \| Entity	35,000
Australia
Segment Reporting Information [Line Items]
Net premiums earned	$ 15.9	$ 17.1	$ 13.0

Amount, after premiums ceded to other entities and premiums assumed by the entity, of property and casualty premiums earned.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.1)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b),(c)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6486662&loc=d3e27175-158546

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration