Proceeds from Sales of Available for Sale Securities and Gross Realized Gains and Losses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 128.8	$ 297.3	$ 408.0	$ 403.8
Gross Gains	1.9	11.7	6.6	15.9
Gross Losses	0.5	0.2	1.6	1.0
Equity Securities
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	2.8	1.8	3.5	1.8
Gross Gains	0.5	0.4	0.5	0.4
Gross Losses			$ 0.2

The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 21

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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