Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 44,570,400,000		$ 44,570,400,000		$ 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	184,500,000		184,500,000
Carrying Amount of Variable Interest Entity Investments	451,000,000		451,000,000
Unfunded Commitment to Fund Investment Partnerships	8,100,000		8,100,000
Commitment to Fund Partnership Equity Investments	149,200,000		149,200,000
Other Long-term Investments	585,000,000	$ 531,800,000	585,000,000	$ 531,800,000	$ 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
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	$ 12,900,000	31,200,000	$ 13,000,000	$ 25,200,000
Interest Income on Mortgage Loans Subsequent to Impairment	200,000	400,000	400,000	700,000
Committments to Fund Commercial Mortgage Loans	$ 113,000,000		$ 113,000,000
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	$ 221,800,000		$ 221,800,000		$ 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	44,570,400,000		44,570,400,000		45,064,900,000
Accumulated Gross Unrealized Gain on Securities	5,117,100,000		5,117,100,000		6,386,300,000
Accumulated Gross Unrealized Loss on Securities	215,900,000		215,900,000		124,800,000
Collateral Pledged to Federal Home Loan Bank (FHLB)	336,800,000		336,800,000
Mortgage Loans as Collateral
Schedule of Investments
Collateral Pledged to Federal Home Loan Bank (FHLB)	101,000,000		101,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	34,900,000		34,900,000		58,400,000
Securities Received as Collateral	196,500,000		196,500,000		128,500,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Unpaid Principal Balance of Impaired Mortgage Loan	14,600,000		14,600,000		14,600,000
Impaired Mortgage Loans, Related Allowance					1,500,000
Impaired Mortgage Loans, with Related Allowance, Recorded Investment	12,600,000		12,600,000		$ 13,100,000
Provision for Loan Losses	500,000	$ 0	$ 500,000	3,000,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure			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	146,200,000		146,200,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	1,400,000		1,400,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	220,700,000		220,700,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	230,300,000		230,300,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	40,910,700,000		40,910,700,000
Accumulated Gross Unrealized Gain on Securities	4,967,500,000		4,967,500,000
Accumulated Gross Unrealized Loss on Securities	$ 140,900,000		$ 140,900,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	65.30%		65.30%
Number of Fixed Maturity Securities in an Unrealized Loss Position	213		213
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	8		8
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 3,659,700,000		$ 3,659,700,000
Accumulated Gross Unrealized Gain on Securities	149,600,000		149,600,000
Accumulated Gross Unrealized Loss on Securities	$ 75,000,000		$ 75,000,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	34.70%		34.70%
Number of Fixed Maturity Securities in an Unrealized Loss Position	82		82
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	13		13