Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Schedule of Investments
Fair value of fixed maturity securities	$ 44,715,200,000		$ 44,715,200,000		$ 42,344,400,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0		0		0
Commitment to fund private placement fixed maturity securities	117,000,000		117,000,000
Carrying amount of variable interest entity investments	469,800,000		469,800,000
Unfunded commitment to fund Investment Partnerships	13,700,000		13,700,000
Commitment to fund partnership Equity Investments	128,400,000		128,400,000
Other Long-term Investments	558,800,000		558,800,000		566,000,000
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 partnership	nor did we fund any amounts to the partnership
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 years
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans			0
Recorded Investment in Financing Receivables on Nonaccrual Status	18,100,000		18,100,000
Number of Financing Receivables on Nonaccrual Status	1		1		0
Average Recorded Investment in Impaired Financing Receivable	31,200,000	13,100,000	27,200,000	15,500,000
Interest Income on Impaired Financing Receivable, Accrual Method	200,000	200,000	900,000	600,000
Committments to Fund Commercial Mortgage Loans	80,900,000		80,900,000
Repurchase agreements - Typical Days outstanding			30 days
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%		102.00%
Fixed Maturity Securities Loaned to Third Parties	174,100,000		174,100,000		201,600,000
Collateral Received in Form of Cash	70,200,000		70,200,000		76,500,000
Securities Received as Collateral	110,400,000		110,400,000		132,900,000
Outstanding Repurchase Agreements	0		0		0
Special Purpose Entity | Bonds
Schedule of Investments
Fair value of fixed maturity securities	142,200,000		142,200,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	1,600,000		1,600,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying amount of variable interest entity investments	293,800,000		293,800,000
Equity Method Investments
Schedule of Investments
Carrying amount of variable interest entity investments	176,000,000		176,000,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair value of fixed maturity securities	41,198,300,000		41,198,300,000
Fair value of fixed maturity securities, gross unrealized gain			5,719,900,000
Fair Value of fixed maturity securities, gross unrealized loss			47,600,000
Fixed maturity securities in unrealized loss position, percent of total	56.30%		56.30%
Number of fixed maturity securities that were in an unrealized loss position	94		94
Number of fixed maturity securities in an unrealized loss position continuously for over one year	48		48
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair value of fixed maturity securities	3,516,900,000		3,516,900,000
Fair value of fixed maturity securities, gross unrealized gain			159,500,000
Fair Value of fixed maturity securities, gross unrealized loss			$ 37,000,000
Fixed maturity securities in unrealized loss position, percent of total	43.70%		43.70%
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	14		14