Investments - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
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	6,850,200,000		7,277,500,000
Fair Value of fixed maturity securities, gross unrealized loss	46,300,000		56,000,000
Defaults on securities not OTTI	0
Commitment to fund private placement fixed maturity securities	153,000,000
Carrying amount of variable interest entity investments	431,700,000
Unfunded commitment to fund Investment Partnerships	75,700,000
Commitment to fund partnership Equity Investments	107,800,000
Other Long-term Investments	592,800,000		625,000,000
Commitments to Fund Special Purpose Entities	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	17,400,000
Impaired Financing Receivable, Interest Income, Accrual Method	200,000	200,000
Foreclosed Real Estate Expense	0	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0		0
Financing Receivable, Recorder Investment, Nonaccrual status	1		1
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	4,300,000		4,300,000
Committments to Fund Commercial Mortgage Loans	48,200,000
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	234,000,000		452,800,000
Cash Collateral for Borrowed Securities	236,300,000		455,800,000
Securities Received as Collateral	0		0
Outstanding Repurchase Agreements	0		0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Amount of days past due-DelinquentMortgageLoan	90 days		90 days
Off Balance Sheet Amount [Member]
Schedule of Investments [Line Items]
Securities Received as Collateral	6,500,000		14,500,000
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	326,300,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	105,400,000
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	141,800,000
Partnership Interest
Schedule of Investments [Line Items]
Other Long-term Investments	6,700,000
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	41,295,100,000
Fair value of fixed maturity securities, gross unrealized gain	6,647,800,000
Fair Value of fixed maturity securities, gross unrealized loss	27,800,000
Fixed maturity securities in unrealized loss position, percent of total	60.00%
Number of fixed maturity securities that were in an unrealized loss position	64
Number of fixed maturity securities in an unrealized loss position continuously for over one year	15
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	3,141,400,000
Fair value of fixed maturity securities, gross unrealized gain	202,400,000
Fair Value of fixed maturity securities, gross unrealized loss	18,500,000
Fixed maturity securities in unrealized loss position, percent of total	40.00%
Number of fixed maturity securities that were in an unrealized loss position	33
Number of fixed maturity securities in an unrealized loss position continuously for over one year	14
Commercial Real Estate Portfolio Segment [Member]
Schedule of Investments [Line Items]
Financing Receivable, Modifications, Number of Contracts	0
Financing Receivable, Modifications, Subsequent Default, Number of Contracts			1
Financing Receivable, Modifications, Subsequent Default, Recorded Investment			$ 4,200,000