Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Investments
Fair value of Fixed Maturity Securities	$ 44,217,300,000	$ 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	30,000,000
Carrying amount of variable interest entity investments	509,300,000
Commitments to Qualified Affordable Housing Project Investments	4,600,000
Unfunded Unconditional Commitment to Fund Private Equity Partnerships	400,000
Commitment to fund partnership Equity Investments	251,900,000
Other Long-term Investments	631,500,000	$ 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	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
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans	0
Average Recorded Investment of Impaired Mortgage Loans	$ 2,200,000	$ 8,600,000	26,700,000
Interest Income on Mortgage Loans Subsequent to Impairment		600,000	1,000,000
Committments to Fund Commercial Mortgage Loans	$ 32,100,000
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	$ 178,500,000	181,600,000
Outstanding Repurchase Agreements	0	0
Federal Home Loan Bank Stock	31,600,000	30,900,000
Advances from FHLB	350,000,000	350,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	29,900,000	29,000,000
Securities Received as Collateral	155,300,000	159,300,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Unpaid Principal Balance of Impaired Mortgage Loan			14,600,000
Financing Receivable, Allowance for Credit Losses	0	0	1,500,000	$ 1,500,000
Impaired Financing Receivable, with Related Allowance, Recorded Investment			13,100,000
Provisions	0	500,000	3,000,000
Special Purpose Entity | Bonds
Schedule of Investments
Fair value of Fixed Maturity Securities	151,900,000	148,000,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	1,000,000	900,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying amount of variable interest entity investments	165,200,000
Equity Method Investments
Schedule of Investments
Carrying amount of variable interest entity investments	344,100,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair value of Fixed Maturity Securities	40,819,300,000
Accumulated Gross Unrealized Gain on Securities	4,764,500,000
Accumulated Gross Unrealized Loss on Securities	$ 151,800,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	69.40%
Number of fixed maturity securities that were in an unrealized loss position	332
Number of fixed maturity securities in an unrealized loss position continuously for over one year	16
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Fair value of Fixed Maturity Securities	$ 3,398,000,000
Accumulated Gross Unrealized Gain on Securities	118,700,000
Accumulated Gross Unrealized Loss on Securities	$ 66,800,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	30.60%
Number of fixed maturity securities that were in an unrealized loss position	69
Number of fixed maturity securities in an unrealized loss position continuously for over one year	37
Collateralized Mortgage Backed Securities
Schedule of Investments
Collateral Pledged to FHLB	$ 288,500,000	96,000,000
Fixed Maturity Securities
Schedule of Investments
Fair value of Fixed Maturity Securities	44,217,300,000	43,354,400,000
Accumulated Gross Unrealized Gain on Securities	4,883,200,000	4,338,000,000
Accumulated Gross Unrealized Loss on Securities	218,600,000	642,300,000
Collateral Pledged to FHLB	323,700,000	317,200,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Recorded Investment in Mortgage Loan Modifications Due to Foreclosure	5,400,000	$ 0	18,100,000
Provisions			$ 3,000,000
Starmount
Schedule of Investments
Fair value of Fixed Maturity Securities	38,800,000
Available-for-sale Securities, Gross Unrealized Gain (Loss)	$ 1,500,000
Starmount | External Credit Rating, Investment Grade
Schedule of Investments
Number of fixed maturity securities that were in an unrealized loss position	132