Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 45,862,000,000		$ 45,862,000,000		$ 43,354,400,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0		0		0
Commitment to Fund Private Placement Fixed Maturity Securities	88,300,000		88,300,000
Carrying Amount of Variable Interest Entity Investments	467,400,000		467,400,000
Unfunded Commitments to Investment Partnerships	4,800,000		4,800,000
Unfunded Unconditional Commitment to Fund Private Equity Partnerships	500,000		500,000
Commitment to Fund Partnership Equity Investments	243,300,000		243,300,000
Other Long-term Investments	592,200,000		592,200,000		$ 583,000,000
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
Financing Receivable, Modifications, Number of Contracts			1
Impaired Financing Receivable, Recorded Investment	$ 5,400,000		$ 5,400,000
Number of Financing Receivables on Nonaccrual Status					0
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans			0
Average Recorded Investment in Impaired Mortgage Loans	5,400,000	$ 12,900,000	$ 2,700,000	$ 13,000,000
Interest Income on Mortgage Loans Subsequent to Impairment		200,000		400,000
Committments to Fund Commercial Mortgage Loans	$ 77,900,000		$ 77,900,000
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	$ 223,900,000		$ 223,900,000		$ 181,600,000
Outstanding Repurchase Agreements	0		0		0
Federal Home Loan Bank Stock	31,600,000		31,600,000		30,900,000
Federal Home Loan Bank (FHLB) Funding Agreements	349,500,000		349,500,000		350,000,000
Fixed Maturity Securities
Schedule of Investments
Fair Value of Fixed Maturity Securities	45,862,000,000		45,862,000,000		43,354,400,000
Accumulated Gross Unrealized Gain on Securities	6,465,100,000		6,465,100,000		4,338,000,000
Accumulated Gross Unrealized Loss on Securities	165,600,000		165,600,000		642,300,000
Collateral Pledged to Federal Home Loan Bank (FHLB)	369,200,000		369,200,000		317,200,000
Mortgage Loans as Collateral
Schedule of Investments
Collateral Pledged to Federal Home Loan Bank (FHLB)	94,200,000		94,200,000		96,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	73,800,000		73,800,000		29,000,000
Securities Received as Collateral	161,700,000		161,700,000		159,300,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Allowance for Credit Losses	$ 0	2,000,000	$ 0	2,000,000	$ 0
Provision for Loan and Lease Losses		$ 500,000		$ 500,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure	0	0	0	0
Special Purpose Entity | Bonds
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 154,400,000		$ 154,400,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	900,000		900,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	183,600,000		183,600,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	283,800,000		283,800,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	42,309,200,000		42,309,200,000
Accumulated Gross Unrealized Gain on Securities	6,354,700,000		6,354,700,000
Accumulated Gross Unrealized Loss on Securities	$ 36,400,000		$ 36,400,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	22.00%		22.00%
Number of Fixed Maturity Securities in an Unrealized Loss Position	54		54
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	37		37
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 3,552,800,000		$ 3,552,800,000
Accumulated Gross Unrealized Gain on Securities	110,400,000		110,400,000
Accumulated Gross Unrealized Loss on Securities	$ 129,200,000		$ 129,200,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	78.00%		78.00%
Number of Fixed Maturity Securities in an Unrealized Loss Position	78		78
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	50		50