Investments - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, gross unrealized gain	$ 5,680,600,000		$ 6,068,200,000
Fair Value of fixed maturity securities, gross unrealized loss	157,400,000		222,200,000
Defaults on securities not OTTI	0
Commitment to fund private placement fixed maturity securities	81,000,000
Carrying amount of variable interest entity investments	444,200,000
Unfunded commitment to fund Investment Partnerships	152,000,000
Commitment to fund partnership Equity Investments	84,900,000
Other Long-term Investments	635,500,000		639,200,000
Mortgage Loan policy is not to exceed a loan-to-value ratio, percent	75.00%
Mortage Loan Policy Debt Service coverage ratio lower range	125.00%
Mortgage Loans issuance, term years	25
Impaired Financing Receivable, Average Recorded Investment	23,700,000	13,100,000
Interest income recognized on mortgage loans subsequent to impairment	200,000	300,000
Financing Receivable, Recorder Investment, Nonaccrual status	1		1
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	8,900,000		9,400,000
commitment to fund commercial mortgage loans	29,900,000
Repurchase agreements - Typical Days outstanding	30
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%
Cash Collateral for Borrowed Securities	372,200,000		312,300,000
Securities Loaned	374,100,000		319,100,000
Securities Received as Collateral	16,400,000		16,700,000
Outstanding Repurchase Agreements	0.0		0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	90		90
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	339,200,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	105,000,000
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	125,000,000
Nonredeemable Preferred Stock
Schedule of Investments [Line Items]
Other Long-term Investments	100,000
Partnership Interest
Schedule of Investments [Line Items]
Other Long-term Investments	8,600,000
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	39,474,500,000
Fair value of fixed maturity securities, gross unrealized gain	5,533,200,000
Fair Value of fixed maturity securities, gross unrealized loss	88,000,000
Fixed maturity securities in unrealized loss position, percent of total	55.90%
Number of fixed maturity securities that were in an unrealized loss position	88
Number of fixed maturity securities in an unrealized loss position continuously for over one year	30
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	2,936,700,000
Fair value of fixed maturity securities, gross unrealized gain	147,400,000
Fair Value of fixed maturity securities, gross unrealized loss	$ 69,400,000
Fixed maturity securities in unrealized loss position, percent of total	44.10%
Number of fixed maturity securities that were in an unrealized loss position	45
Number of fixed maturity securities in an unrealized loss position continuously for over one year	20