Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) Integer
Schedule of Investments
Number of Impaired Mortgage Loans | Integer					1
Fair Value of Fixed Maturity Securities	$ 44,401,500,000		$ 44,401,500,000		$ 45,064,900,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0		0		0
Commitment to Fund Private Placement Fixed Maturity Securities	47,000,000		47,000,000
Carrying Amount of Variable Interest Entity Investments	438,700,000		438,700,000
Unfunded Commitments to Investment Partnerships	7,200,000		7,200,000
Commitment to Fund Partnership Equity Investments	182,700,000		182,700,000
Other Long-term Investments	590,200,000	$ 516,700,000	590,200,000	$ 516,700,000	$ 545,000,000
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 partnership	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 Financing Receivables on Nonaccrual Status	0		0		0
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans			0
Average Recorded Investment in Impaired Mortgage Loans	$ 8,400,000	$ 31,200,000	$ 11,500,000	$ 27,200,000
Interest Income on Mortgage Loans Subsequent to Impairment	200,000	200,000	600,000	900,000
Committments to Fund Commercial Mortgage Loans	$ 42,400,000		$ 42,400,000
Repurchase agreements - Typical Days Outstanding			30 days
Minimum Percent of Fair Value of Securities Loaned or Securities Purchased Under Repurchase Agreements to be Maintained as Collateral	102.00%		102.00%
Net Securities Borrowed Reported in Balance Sheet	$ 213,600,000		$ 213,600,000		$ 176,500,000
Outstanding Repurchase Agreements	0		0		0
Federal Home Loan Bank Stock	30,900,000		30,900,000
Federal Home Loan Bank (FHLB) Funding Agreements	350,000,000		350,000,000
Fixed Maturity Securities
Schedule of Investments
Fair Value of Fixed Maturity Securities	44,401,500,000		44,401,500,000		45,064,900,000
Accumulated Gross Unrealized Gain on Securities	5,055,700,000		5,055,700,000		6,386,300,000
Accumulated Gross Unrealized Loss on Securities	392,300,000		392,300,000		124,800,000
Collateral Pledged to Federal Home Loan Bank (FHLB)	324,800,000		324,800,000
Mortgage Loans as Collateral
Schedule of Investments
Collateral Pledged to Federal Home Loan Bank (FHLB)	100,100,000		100,100,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	26,100,000		26,100,000		58,400,000
Securities Received as Collateral	194,800,000		194,800,000		128,500,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Unpaid Principal Balance of Impaired Mortgage Loan					14,600,000
Impaired Mortgage Loans, Related Allowance					1,500,000
Impaired Mortgage Loans, with Related Allowance, Recorded Investment					$ 13,100,000
Provision for Loan Losses	$ 0	$ 0	$ 500,000	3,000,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure	0	0	0
Recorded Investment in Mortgage Loan Modifications Due to Foreclosure				18,100,000
Troubled Debt Restructuring Write-Down				$ 3,000,000
Special Purpose Entity | Bonds
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 148,600,000		$ 148,600,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	1,300,000		1,300,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	211,400,000		211,400,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	227,300,000		227,300,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	40,968,000,000		40,968,000,000
Accumulated Gross Unrealized Gain on Securities	4,975,700,000		4,975,700,000
Accumulated Gross Unrealized Loss on Securities	$ 194,500,000		$ 194,500,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	49.60%		49.60%
Number of Fixed Maturity Securities in an Unrealized Loss Position	212		212
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	9		9
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 3,433,500,000		$ 3,433,500,000
Accumulated Gross Unrealized Gain on Securities	80,000,000		80,000,000
Accumulated Gross Unrealized Loss on Securities	$ 197,800,000		$ 197,800,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	50.40%		50.40%
Number of Fixed Maturity Securities in an Unrealized Loss Position	112		112
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	27		27