Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Investments [Line Items]
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	$ 0		$ 0		$ 0
Fair value of fixed maturity securities, gross unrealized gain	4,698,000,000		4,698,000,000		7,277,500,000
Fair Value of fixed maturity securities, gross unrealized loss	287,600,000		287,600,000		56,000,000
Defaults on securities not OTTI			0
Commitment to fund private placement fixed maturity securities	44,200,000		44,200,000
Carrying amount of variable interest entity investments	461,000,000		461,000,000
Unfunded commitment to fund Investment Partnerships	41,900,000		41,900,000
Commitment to fund partnership Equity Investments	120,500,000		120,500,000
Other Long-term Investments	564,500,000		564,500,000		625,000,000
Commitments to Fund Special Purpose Entities	0		0
Amount Funded to Special Purpose Entity Partnerships	nor did we fund any amounts to the partnership	nor did we fund any amounts to the partnerships	nor did we fund any amounts to the partnerships	nor did we fund any amounts to the partnerships
Mortgage Loan policy is not to exceed a loan-to-value ratio, percent	75.00%		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%		125.00%
Mortgage Loans issuance, term years	25		25
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans			0
Impaired Financing Receivable, Average Recorded Investment	13,100,000	15,900,000	15,500,000	18,800,000
Impaired Financing Receivable, Interest Income, Accrual Method	200,000	200,000	600,000	600,000
Foreclosed Real Estate Expense	0	0	0	0
Financing Receivable, Recorder Investment, Nonaccrual status	0		0		1
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due					4,300,000
Committments to Fund Commercial Mortgage Loans	61,600,000		61,600,000
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	254,400,000		254,400,000		452,800,000
Cash Collateral for Borrowed Securities	140,500,000		140,500,000		455,800,000
Securities Received as Collateral	(113,900,000)		(113,900,000)		0
Outstanding Repurchase Agreements	0		0		0
Nonperforming Financing Receivable [Member]
Schedule of Investments [Line Items]
Amount of days past due-DelinquentMortgageLoan	90 days		90 days		90 days
Off Balance Sheet Amount
Schedule of Investments [Line Items]
Securities Received as Collateral	124,100,000		124,100,000		14,500,000
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	315,000,000		315,000,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	146,000,000		146,000,000
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	39,456,000,000		39,456,000,000
Fair value of fixed maturity securities, gross unrealized gain	4,567,900,000		4,567,900,000
Fair Value of fixed maturity securities, gross unrealized loss	221,100,000		221,100,000
Fixed maturity securities in unrealized loss position, percent of total	76.90%		76.90%
Number of fixed maturity securities that were in an unrealized loss position	207		207
Number of fixed maturity securities in an unrealized loss position continuously for over one year	14		14
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	3,141,600,000		3,141,600,000
Fair value of fixed maturity securities, gross unrealized gain	130,100,000		130,100,000
Fair Value of fixed maturity securities, gross unrealized loss	66,500,000		66,500,000
Fixed maturity securities in unrealized loss position, percent of total	23.10%		23.10%
Number of fixed maturity securities that were in an unrealized loss position	62		62
Number of fixed maturity securities in an unrealized loss position continuously for over one year	10		10
Special Purpose Entity | Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	135,600,000		135,600,000
Special Purpose Entity | Partnership Interest
Schedule of Investments [Line Items]
Other Long-term Investments	$ 4,400,000		$ 4,400,000
Commercial Real Estate Portfolio Segment [Member] | Third Quarter [Member] [Domain]
Schedule of Investments [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts	0	0	0	0