Investments - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 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	4,928,900,000		4,928,900,000		7,277,500,000
Fair Value of fixed maturity securities, gross unrealized loss	269,300,000		269,300,000		56,000,000
Defaults on securities not OTTI			0
Commitment to fund private placement fixed maturity securities	48,500,000		48,500,000
Carrying amount of variable interest entity investments	453,400,000		453,400,000
Unfunded commitment to fund Investment Partnerships	56,800,000		56,800,000
Commitment to fund partnership Equity Investments	133,700,000		133,700,000
Other Long-term Investments	583,600,000		583,600,000		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	16,000,000	16,600,000	16,700,000	20,200,000
Impaired Financing Receivable, Interest Income, Accrual Method	200,000	200,000	400,000	400,000
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	119,800,000		119,800,000
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	236,500,000		236,500,000		452,800,000
Cash Collateral for Borrowed Securities	153,600,000		153,600,000		455,800,000
Securities Received as Collateral	(82,900,000)		(82,900,000)		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	92,900,000		92,900,000		14,500,000
Partnership Interests In Tax Credit Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	320,100,000		320,100,000
Equity Method Investments
Schedule of Investments [Line Items]
Carrying amount of variable interest entity investments	133,300,000		133,300,000
External Credit Rating, Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	39,454,600,000		39,454,600,000
Fair value of fixed maturity securities, gross unrealized gain	4,800,600,000		4,800,600,000
Fair Value of fixed maturity securities, gross unrealized loss	215,800,000		215,800,000
Fixed maturity securities in unrealized loss position, percent of total	80.10%		80.10%
Number of fixed maturity securities that were in an unrealized loss position	208		208
Number of fixed maturity securities in an unrealized loss position continuously for over one year	16		16
External Credit Rating, Non Investment Grade
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	3,139,600,000		3,139,600,000
Fair value of fixed maturity securities, gross unrealized gain	128,300,000		128,300,000
Fair Value of fixed maturity securities, gross unrealized loss	53,500,000		53,500,000
Fixed maturity securities in unrealized loss position, percent of total	19.90%		19.90%
Number of fixed maturity securities that were in an unrealized loss position	68		68
Number of fixed maturity securities in an unrealized loss position continuously for over one year	9		9
Special Purpose Entity | Bonds
Schedule of Investments [Line Items]
Fair value of fixed maturity securities	134,400,000		134,400,000
Special Purpose Entity | Partnership Interest
Schedule of Investments [Line Items]
Other Long-term Investments	$ 6,700,000		$ 6,700,000