Investments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2016 USD ($) Integer	Mar. 31, 2015 USD ($) Integer	Dec. 31, 2015 USD ($)
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 44,699,200,000		$ 43,354,400,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0		0
Commitment to Fund Private Placement Fixed Maturity Securities	107,900,000
Carrying Amount of Variable Interest Entity Investments	441,600,000
Unfunded Commitments to Investment Partnerships	4,800,000
Commitment to Fund Partnership Equity Investments	282,800,000
Other Long-term Investments	572,200,000		$ 583,000,000
Commitments to Fund Special Purpose Entities	$ 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%
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 years
Number of Financing Receivables on Nonaccrual Status	0		0
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans	0
Number of Impaired Mortgage Loans | Integer	0	1
Average Recorded Investment in Impaired Mortgage Loans		$ 13,100,000
Interest Income on Mortgage Loans Subsequent to Impairment	$ 0	200,000
Committments to Fund Commercial Mortgage Loans	$ 83,500,000
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%
Net Securities Borrowed Reported in Balance Sheet	$ 194,300,000		$ 181,600,000
Outstanding Repurchase Agreements	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	44,699,200,000		43,354,400,000
Accumulated Gross Unrealized Gain on Securities	5,319,000,000		4,338,000,000
Accumulated Gross Unrealized Loss on Securities	424,500,000		642,300,000
Collateral Pledged to Federal Home Loan Bank (FHLB)	318,600,000		317,200,000
Mortgage Loans as Collateral
Schedule of Investments
Collateral Pledged to Federal Home Loan Bank (FHLB)	95,100,000		96,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	48,800,000		29,000,000
Securities Received as Collateral	152,200,000		159,300,000
Commercial Portfolio Segment [Member]
Schedule of Investments
Financing Receivable, Allowance for Credit Losses	0	1,500,000	$ 0
Commercial Real Estate Portfolio Segment
Schedule of Investments
Provision for Loan Losses	0	0
Financing Receivable, Allowance for Credit Losses, Write-downs	$ 0	$ 0
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure	0	0
Special Purpose Entity | Bonds
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 151,900,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	1,000,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	192,900,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	248,700,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	41,305,800,000
Accumulated Gross Unrealized Gain on Securities	5,220,300,000
Accumulated Gross Unrealized Loss on Securities	$ 174,900,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	41.20%
Number of Fixed Maturity Securities in an Unrealized Loss Position	145
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	27
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 3,393,400,000
Accumulated Gross Unrealized Gain on Securities	98,700,000
Accumulated Gross Unrealized Loss on Securities	$ 249,600,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	58.80%
Number of Fixed Maturity Securities in an Unrealized Loss Position	84
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	34