Investments - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of Investments [Line Items]
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	$ 0	$ 0
Fair value of fixed maturity securities, gross unrealized gain	7,277,500,000	6,068,200,000
Fair Value of fixed maturity securities, gross unrealized loss	56,000,000	222,200,000
Defaults on securities not OTTI	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0
Commitment to fund private placement fixed maturity securities	29,000,000
Carrying amount of variable interest entity investments	447,100,000
Unfunded commitment to fund Investment Partnerships	83,700,000
Commitment to fund partnership Equity Investments	71,300,000
Other Long-term Investments	625,000,000	639,200,000
Commitments to fund special purpose entity	0
Mortgage Loan policy is not to exceed a loan-to-value ratio, percent	75.00%
Loan to Value Ratio Update Frequency	3 years
Mortgage Loan Inspection Frequency	2 years
Mortage Loan Policy Debt Service coverage ratio lower range	125.00%
Mortgage Loans issuance, term years	25
Impaired Financing Receivable, Average Recorded Investment	19,100,000	21,300,000	22,900,000
Impaired Financing Receivable, Interest Income, Accrual Method	800,000	800,000	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0
Financing Receivable, Recorder Investment, Nonaccrual status	1	1
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	4,300,000	9,400,000
Committments to Fund Commercial Mortgage Loans	47,300,000
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%
Securities Loaned	452,800,000	319,100,000
Cash Collateral for Borrowed Securities	455,800,000	312,300,000
Securities Received as Collateral	14,500,000	16,700,000
Outstanding Repurchase Agreements	0.0	0.0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Amount of days past due-DelinquentMortgageLoan	90 days	90 days
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	331,500,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	115,600,000
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	140,800,000
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	41,813,700,000
Fair value of fixed maturity securities, gross unrealized gain	7,076,200,000
Fair Value of fixed maturity securities, gross unrealized loss	23,400,000
Fixed maturity securities in unrealized loss position, percent of total	41.80%
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	16
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	3,159,300,000
Fair value of fixed maturity securities, gross unrealized gain	201,300,000
Fair Value of fixed maturity securities, gross unrealized loss	$ 32,600,000
Fixed maturity securities in unrealized loss position, percent of total	58.20%
Number of fixed maturity securities that were in an unrealized loss position	29
Number of fixed maturity securities in an unrealized loss position continuously for over one year	18