Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Schedule of Investments [Line Items]
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent			$ 0		$ 0
Fair value of fixed maturity securities, gross unrealized gain	7,378,400,000		7,378,400,000		6,068,200,000
Fair Value of fixed maturity securities, gross unrealized loss	91,000,000		91,000,000		222,200,000
Defaults on securities not OTTI	0
Commitment to fund private placement fixed maturity securities	125,000,000		125,000,000
Carrying amount of variable interest entity investments	452,100,000		452,100,000
Unfunded commitment to fund Investment Partnerships	112,200,000		112,200,000
Commitment to fund partnership Equity Investments	73,100,000		73,100,000
Other Long-term Investments	650,900,000		650,900,000		639,200,000
Commitment to fund SPE - consolidated	0		0
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
Impaired Financing Receivable, Average Recorded Investment	15,900,000	26,400,000	18,800,000	20,900,000
Interest income recognized on mortgage loans subsequent to impairment	200,000	200,000	600,000	600,000
realized loss on foreclosure			0
Financing Receivable, Recorder Investment, Nonaccrual status	1		1		1
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	4,200,000		4,200,000
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due					9,400,000
commitment to fund commercial mortgage loans	117,100,000		117,100,000
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%		102.00%
Securities Loaned	457,300,000		457,300,000		319,100,000
Cash Collateral for Borrowed Securities	458,300,000		458,300,000		312,300,000
Securities Received as Collateral	15,700,000		15,700,000		16,700,000
Outstanding Repurchase Agreements	0.0		0.0		0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Amount of days past due-DelinquentMortgageLoan	60		60		90
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	337,400,000		337,400,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	114,700,000		114,700,000
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, investment grade	140,200,000		140,200,000
Nonredeemable Preferred Stock
Schedule of Investments [Line Items]
Other Long-term Investments	100,000		100,000
Partnership Interest
Schedule of Investments [Line Items]
Other Long-term Investments	8,200,000		8,200,000
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, investment grade	41,593,800,000		41,593,800,000
Fair value of fixed maturity securities, gross unrealized gain	7,193,500,000		7,193,500,000
Fair Value of fixed maturity securities, gross unrealized loss	38,000,000		38,000,000
Fixed maturity securities in unrealized loss position, percent of total	41.80%		41.80%
Number of fixed maturity securities that were in an unrealized loss position	36		36
Number of fixed maturity securities in an unrealized loss position continuously for over one year	22		22
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, investment grade	3,150,900,000		3,150,900,000
Fair value of fixed maturity securities, gross unrealized gain	184,900,000		184,900,000
Fair Value of fixed maturity securities, gross unrealized loss	$ 53,000,000		$ 53,000,000
Fixed maturity securities in unrealized loss position, percent of total	58.20%		58.20%
Number of fixed maturity securities that were in an unrealized loss position	35		35
Number of fixed maturity securities in an unrealized loss position continuously for over one year	20		20