Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Investments
Fair value of Fixed Maturity Securities	$ 43,354,400,000	$ 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	56,000,000
Carrying amount of variable interest entity investments	442,600,000
Commitments to Qualified Affordable Housing Project Investments	5,000,000
Commitment to fund partnership Equity Investments	168,500,000
Other Long-term Investments	583,000,000	$ 545,000,000
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	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	three
Mortgage Loan Inspection Frequency	two
Mortage Loan Policy Debt Service coverage ratio lower range	125.00%
Mortgage Loans issuance, term years	25
Foreclosed Real Estate Expense	$ 0	$ 0	$ 0
Financing Receivable, Recorder Investment, Nonaccrual status	0	0	0
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans	0
Average Recorded Investment of Impaired Mortgage Loans	$ 8,600,000	$ 26,700,000	$ 14,900,000
Interest Income on Mortgage Loans Subsequent to Impairment	600,000	1,000,000	800,000
Committments to Fund Commercial Mortgage Loans	$ 67,000,000
Repurchase agreements - Typical Days outstanding	30
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%
Gross Securities Borrowed	$ 181,600,000	176,500,000
Outstanding Repurchase Agreements	0	0
Federal Home Loan Bank Stock	30,900,000
Advances from FHLB	350,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	29,000,000	58,400,000
Securities Received as Collateral	159,300,000	128,500,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Unpaid Principal Balance of Impaired Mortgage Loan		14,600,000
Impaired Financing Receivable, Related Allowance		1,500,000
Impaired Financing Receivable, with Related Allowance, Recorded Investment		13,100,000
Provisions	500,000	3,000,000	0
Special Purpose Entity | Bonds
Schedule of Investments
Fair value of Fixed Maturity Securities	148,000,000	143,900,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	900,000	1,400,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying amount of variable interest entity investments	202,100,000
Equity Method Investments
Schedule of Investments
Carrying amount of variable interest entity investments	240,500,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair value of Fixed Maturity Securities	40,056,300,000
Accumulated Gross Unrealized Gain on Securities	4,264,400,000
Accumulated Gross Unrealized Loss on Securities	$ 360,400,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	56.10%
Number of fixed maturity securities that were in an unrealized loss position	269
Number of fixed maturity securities in an unrealized loss position continuously for over one year	14
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Fair value of Fixed Maturity Securities	$ 3,298,100,000
Accumulated Gross Unrealized Gain on Securities	73,600,000
Accumulated Gross Unrealized Loss on Securities	$ 281,900,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	43.90%
Number of fixed maturity securities that were in an unrealized loss position	100
Number of fixed maturity securities in an unrealized loss position continuously for over one year	32
Collateralized Mortgage Backed Securities
Schedule of Investments
Collateral Pledged to FHLB	$ 96,000,000
Fixed Maturity Securities
Schedule of Investments
Fair value of Fixed Maturity Securities	43,354,400,000	45,064,900,000
Accumulated Gross Unrealized Gain on Securities	4,338,000,000	6,386,300,000
Accumulated Gross Unrealized Loss on Securities	642,300,000	124,800,000
Collateral Pledged to FHLB	317,200,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Recorded Investment in Mortgage Loan Modifications Due to Foreclosure	$ 0	18,100,000	$ 4,300,000
Troubled Debt Restructuring Write-down		$ 3,000,000