Investments - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 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
Fair value of fixed maturity securities, gross unrealized gain	6,572,700,000		6,572,700,000		6,068,200,000
Fair Value of fixed maturity securities, gross unrealized loss	167,800,000		167,800,000		222,200,000
Defaults on securities not OTTI	0
Commitment to fund private placement fixed maturity securities	142,300,000		142,300,000
Carrying amount of variable interest entity investments	447,200,000		447,200,000
Unfunded commitment to fund Investment Partnerships	122,000,000		122,000,000
Commitment to fund partnership Equity Investments	75,000,000		75,000,000
Other Long-term Investments	655,000,000		655,000,000		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	16,600,000	19,800,000	20,200,000	18,100,000
Interest income recognized on mortgage loans subsequent to impairment	200,000	100,000	400,000	400,000
realized loss on foreclosure			0
Financing Receivable, Recorder Investment, Nonaccrual status	1		1		1
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	900,000		900,000
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due					9,400,000
commitment to fund commercial mortgage loans	32,500,000		32,500,000
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	463,900,000		463,900,000		319,100,000
Cash Collateral for Borrowed Securities	467,100,000		467,100,000		312,300,000
Securities Received as Collateral	14,200,000		14,200,000		16,700,000
Outstanding Repurchase Agreements	0.0		0.0		0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	60		60
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	333,700,000		333,700,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	113,500,000		113,500,000
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, investment grade	131,500,000		131,500,000
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	8,100,000		8,100,000
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, investment grade	40,494,100,000		40,494,100,000
Fair value of fixed maturity securities, gross unrealized gain	6,438,000,000		6,438,000,000
Fair Value of fixed maturity securities, gross unrealized loss	84,700,000		84,700,000
Fixed maturity securities in unrealized loss position, percent of total	50.50%		50.50%
Number of fixed maturity securities that were in an unrealized loss position	69		69
Number of fixed maturity securities in an unrealized loss position continuously for over one year	28		28
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities, investment grade	3,022,800,000		3,022,800,000
Fair value of fixed maturity securities, gross unrealized gain	134,700,000		134,700,000
Fair Value of fixed maturity securities, gross unrealized loss	$ 83,100,000		$ 83,100,000
Fixed maturity securities in unrealized loss position, percent of total	49.50%		49.50%
Number of fixed maturity securities that were in an unrealized loss position	45		45
Number of fixed maturity securities in an unrealized loss position continuously for over one year	23		23