Investments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, gross unrealized gain	$ 6,068,200,000	$ 3,726,400,000
Fair Value of fixed maturity securities, gross unrealized loss	222,200,000	237,400,000
Defaults on securities not OTTI	0
Commitment to fund private placement fixed maturity securities	35,000,000
Carrying amount of variable interest entity investments	428,300,000
Unfunded commitment to fund Investment Partnerships	160,600,000
Commitment to fund partnership Equity Investments	65,400,000
Other Long-term Investments	639,200,000	529,300,000
Commitments to fund special purpose entity	500,000
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	21,300,000	22,900,000	7,400,000
Interest income recognized on mortgage loans subsequent to impairment	800,000	0	100,000
Financing Receivable, Recorder Investment, Nonaccrual status	1	0
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	9,400,000
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	312,300,000	0
Securities Loaned	319,100,000
Securities Received as Collateral	16,700,000
Outstanding Repurchase Agreements	0.0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	90
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	329,900,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	98,400,000
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	121,300,000
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	8,000,000
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	39,675,800,000
Fair value of fixed maturity securities, gross unrealized gain	5,951,600,000
Fair Value of fixed maturity securities, gross unrealized loss	140,300,000
Fixed maturity securities in unrealized loss position, percent of total	63.10%
Number of fixed maturity securities that were in an unrealized loss position	92
Number of fixed maturity securities in an unrealized loss position continuously for over one year	42
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	2,810,900,000
Fair value of fixed maturity securities, gross unrealized gain	116,600,000
Fair Value of fixed maturity securities, gross unrealized loss	$ 81,900,000
Fixed maturity securities in unrealized loss position, percent of total	36.90%
Number of fixed maturity securities that were in an unrealized loss position	55
Number of fixed maturity securities in an unrealized loss position continuously for over one year	22