Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Schedule of Investments [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 19.8	$ 18.1
Fair value of fixed maturity securities, gross unrealized gain	4,088.1	4,088.1	3,726.4
Fair Value of fixed maturity securities, gross unrealized loss	172.2	172.2	237.4
Unfunded commitment to fund tax credit partnership investments	177.8	177.8
Commitment to fund private equity investments	61.2	61.2
Commitments to fund special purpose entity	1.9	1.9
Mortgage Loan policy is not to exceed a loan-to-value ratio, percent	75.00%	75.00%
Mortage Loan Policy Debt Service coverage ratio lower range	125.00%	125.00%
Mortgage Loans issuance, term years	25	25
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%	102.00%
Cash Collateral for Borrowed Securities	140.4	140.4
Carrying amount of variable interest entity investments	414.3	414.3
Other Long-term Investments	573.9	573.9	529.3
Interest income recognized on mortgage loans subsequent to impairment	0.1	0.4
Recorded Investment	9.8	9.8
Amount of days past due-DelinquentMortgageLoan	90	90
Financing Receivable, Recorded Investment, Past Due	10.1	10.1
commitment to fund commercial mortgage loans	30.1	30.1
Securities Loaned	136.1	136.1
Commitment to fund private placement fixed maturity securities	105.1	105.1
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	37,945.8	37,945.8
Fair value of fixed maturity securities, gross unrealized gain	3,946.1	3,946.1
Fair Value of fixed maturity securities, gross unrealized loss	135.7	135.7
Fixed maturity securities in unrealized loss position, percent of total	78.80%	78.80%
Number of fixed maturity securities that were in an unrealized loss position	165	165
Number of fixed maturity securities in an unrealized loss position continuously for over one year	47	47
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	2,822.6	2,822.6
Fair value of fixed maturity securities, gross unrealized gain	142.0	142.0
Fair Value of fixed maturity securities, gross unrealized loss	36.5	36.5
Fixed maturity securities in unrealized loss position, percent of total	21.20%	21.20%
Number of fixed maturity securities that were in an unrealized loss position	42	42
Number of fixed maturity securities in an unrealized loss position continuously for over one year	22	22
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	315.4	315.4
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	98.9	98.9
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	112.0	112.0
Nonredeemable Preferred Stock
Schedule of Investments [Line Items]
Other Long-term Investments	0.1	0.1
Partnership Interest
Schedule of Investments [Line Items]
Other Long-term Investments	$ 11.1	$ 11.1

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