Mortgage Loans, Sorted by Applicable Credit Quality Indicators, Internal rating (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,730.4	$ 1,712.7
Aa
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	11.3	11.5
A
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	660.8	659.4
Baa
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	1,005.6	994.5
Ba
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	39.6	34.2
B
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 13.1	$ 13.1

The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 60

-Paragraph 47

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1(c))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph c

-Article 7

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