Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Schedule of Investments [Line Items]
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	$ 0		$ 0		$ 3,900,000
Fair value of fixed maturity securities, gross unrealized gain	5,957,600,000		5,957,600,000		3,726,400,000
Fair Value of fixed maturity securities, gross unrealized loss	269,600,000		269,600,000		237,400,000
Defaults on securities not OTTI			0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	0	0	8,500,000	0
Commitment to fund private placement fixed maturity securities	28,700,000		28,700,000
Carrying amount of variable interest entity investments	405,100,000		405,100,000
Unfunded commitment to fund tax credit partnership investments	158,600,000		158,600,000
Commitment to fund private equity investments	57,300,000		57,300,000
Other Long-term Investments	611,100,000		611,100,000		529,300,000
Commitments to fund special purpose entity	500,000		500,000
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	26,400,000		20,900,000
Interest income recognized on mortgage loans subsequent to impairment	200,000		600,000
Real Estate Acquired Through Foreclosure	9,800,000		9,800,000
realized loss on foreclosure			0
Financing Receivable, Recorder Investment, Nonaccrual status	2		2		0
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	10,100,000		10,100,000
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	9,500,000		9,500,000
commitment to fund commercial mortgage loans	17,800,000		17,800,000
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%		102.00%
Cash Collateral for Borrowed Securities	342,400,000		342,400,000
Securities Loaned	344,500,000		344,500,000
Securities Received as Collateral	15,400,000		15,400,000
Repurchase agreements - Typical Days outstanding			30
Outstanding Repurchase Agreements	0.0		0.0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due	90		90
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	30		30
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	39,732,200,000		39,732,200,000
Fair value of fixed maturity securities, gross unrealized gain	5,889,000,000		5,889,000,000
Fair Value of fixed maturity securities, gross unrealized loss	146,000,000		146,000,000
Fixed maturity securities in unrealized loss position, percent of total	54.20%		54.20%
Number of fixed maturity securities that were in an unrealized loss position	96		96
Number of fixed maturity securities in an unrealized loss position continuously for over one year	44		44
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	2,702,500,000		2,702,500,000
Fair value of fixed maturity securities, gross unrealized gain	68,600,000		68,600,000
Fair Value of fixed maturity securities, gross unrealized loss	123,600,000		123,600,000
Fixed maturity securities in unrealized loss position, percent of total	45.80%		45.80%
Number of fixed maturity securities that were in an unrealized loss position	82		82
Number of fixed maturity securities in an unrealized loss position continuously for over one year	23		23
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	310,200,000		310,200,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	94,900,000		94,900,000
Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	122,800,000		122,800,000
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	$ 8,500,000		$ 8,500,000