Investments - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Schedule of Investments
Fair value of fixed maturity securities	$ 43,666,800,000		$ 42,344,400,000
Fair value of fixed maturity securities, gross unrealized gain	5,290,600,000		4,358,700,000
Fair Value of fixed maturity securities, gross unrealized loss	139,800,000		303,900,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0		0
Commitment to fund private placement fixed maturity securities	60,000,000
Carrying amount of variable interest entity investments	473,000,000
Unfunded commitment to fund Investment Partnerships	17,900,000
Commitment to fund partnership Equity Investments	144,700,000
Other Long-term Investments	565,000,000		566,000,000
Commitments to Fund Special Purpose Entities	0
Amount Funded to Special Purpose Entity 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%
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%
Mortgage Loans issuance, term years	25
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans	0	0
Number of Troubled Debt Restructurings During Period	1
Pre-Modification Recorded Investment in Troubled Debt Restructuring	21,100,000
Impaired Financing Receivable, Related Allowance	3,000,000
Financing Receivable, Recorded Investment, Nonaccrual status	0		0
Average Recorded Investment in Impaired Financing Receivable	19,100,000	17,400,000
Interest Income on Impaired Financing Receivable, Accrual Method	300,000	200,000
Committments to Fund Commercial Mortgage Loans	21,700,000
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%
Fixed Maturity Securities Loaned to Third Parties	187,400,000		201,600,000
Collateral Received in Form of Cash	61,600,000		76,500,000
Securities Received as Collateral	132,500,000		132,900,000
Outstanding Repurchase Agreements	0		0
Nonperforming Financing Receivable
Schedule of Investments
Amount of days past due-DelinquentMortgageLoan	90 days
Special Purpose Entity | Bonds
Schedule of Investments
Fair value of fixed maturity securities	138,100,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	4,400,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying amount of variable interest entity investments	304,800,000
Equity Method Investments
Schedule of Investments
Carrying amount of variable interest entity investments	168,200,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair value of fixed maturity securities	40,372,300,000
Fair value of fixed maturity securities, gross unrealized gain	5,104,700,000
Fair Value of fixed maturity securities, gross unrealized loss	113,500,000
Fixed maturity securities in unrealized loss position, percent of total	81.20%
Number of fixed maturity securities that were in an unrealized loss position	148
Number of fixed maturity securities in an unrealized loss position continuously for over one year	16
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair value of fixed maturity securities	3,294,500,000
Fair value of fixed maturity securities, gross unrealized gain	185,900,000
Fair Value of fixed maturity securities, gross unrealized loss	$ 26,300,000
Fixed maturity securities in unrealized loss position, percent of total	18.80%
Number of fixed maturity securities that were in an unrealized loss position	38
Number of fixed maturity securities in an unrealized loss position continuously for over one year	10