Investments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2017 USD ($) Integer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Integer	Sep. 30, 2016 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Investments
Fair value of Fixed Maturity Securities			$ 45,457,800,000	$ 44,217,300,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			0.0	0
Commitment to fund private placement fixed maturity securities			41,800,000
Carrying amount of variable interest entity investments			535,400,000	509,300,000
Commitments to Qualified Affordable Housing Project Investments			3,700,000
Commitment to fund partnership Equity Investments			356,900,000
Other Long-term Investments			646,800,000	631,500,000
Gain (Loss) on Investments, Excluding Other than Temporary Impairments			$ 48,400,000	54,700,000	$ (11,400,000)
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
Impaired Financing Receivable, Recorded Investment						$ 5,400,000
Financing Receivable, Recorder Investment, Nonaccrual status			0	0
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans			0
Financing Receivable, Allowance for Credit Losses		$ 2,000,000			0
Financing Receivable, Allowance for Credit Losses, Write-downs					2,000,000
Average Recorded Investment of Impaired Mortgage Loans			$ 0	$ 2,200,000	8,600,000
Interest Income on Mortgage Loans Subsequent to Impairment			0	0	$ 600,000
Committments to Fund Commercial Mortgage Loans			$ 45,300,000
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			$ 159,200,000	178,500,000
Outstanding Repurchase Agreements			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
Federal Home Loan Bank Stock			34,100,000	$ 31,600,000
Advances from FHLB			350,000,000	350,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities			30,500,000	29,900,000
Securities Received as Collateral			135,600,000	155,300,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Unpaid Principal Balance of Impaired Mortgage Loan							$ 14,600,000
Provisions		$ 500,000	0	0
Collateralized Mortgage Backed Securities
Schedule of Investments
Collateral Pledged to FHLB			331,500,000	288,500,000
Fixed Maturity Securities
Schedule of Investments
Fair value of Fixed Maturity Securities			45,457,800,000	44,217,300,000
Accumulated Gross Unrealized Gain on Securities			5,800,800,000	4,883,200,000
Accumulated Gross Unrealized Loss on Securities			123,500,000	218,600,000
Collateral Pledged to FHLB			$ 213,300,000	323,700,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | Integer			0		0
Special Purpose Entity | Bonds
Schedule of Investments
Fair value of Fixed Maturity Securities			$ 154,100,000	151,900,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments				1,000,000
Gain (Loss) on Investments, Excluding Other than Temporary Impairments	$ (200,000)
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying amount of variable interest entity investments			128,200,000	165,200,000
Equity Method Investments
Schedule of Investments
Carrying amount of variable interest entity investments			407,200,000	$ 344,100,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair value of Fixed Maturity Securities			42,093,500,000
Accumulated Gross Unrealized Gain on Securities			5,628,100,000
Accumulated Gross Unrealized Loss on Securities			$ 66,500,000
Percent of Fixed Maturity Securities in Unrealized Loss Position			53.80%
Number of fixed maturity securities that were in an unrealized loss position			226
Number of fixed maturity securities in an unrealized loss position continuously for over one year			134
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Fair value of Fixed Maturity Securities			$ 3,364,300,000
Accumulated Gross Unrealized Gain on Securities			172,700,000
Accumulated Gross Unrealized Loss on Securities			$ 57,000,000
Percent of Fixed Maturity Securities in Unrealized Loss Position			46.20%
Number of fixed maturity securities that were in an unrealized loss position			47
Number of fixed maturity securities in an unrealized loss position continuously for over one year			21