Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed Maturity Securities	$ 44,217.3	$ 43,354.4
Mortgage Loans	2,038.9	1,883.6
Policy Loans	3,463.2	3,395.4
Other Long-term Investments
Derivative Assets	32.7	49.8
Equity Securities	1.2	1.4
Policyholders' Funds
Long-term Debt	2,999.4	2,449.4
Advances from FHLB	350.0	350.0
Other Liabilities
Derivative Liabilities	99.5	137.8
Carrying Amount
Assets
Fixed Maturity Securities	44,217.3	43,354.4
Mortgage Loans	2,038.9	1,883.6
Policy Loans	3,463.2	3,395.4
Other Long-term Investments
Derivative Assets	32.7	49.8
Equity Securities	1.2	1.4
Miscellaneous Long-Term Investments	541.9	474.4
Policyholders' Funds
Deferred Annuity Products	597.4	608.8
Supplementary Contracts without Life Contingencies	608.8	641.1
Short-term Debt	0.0	352.0
Long-term Debt	2,999.4	2,449.4
Advances from FHLB	350.0	350.0
Other Liabilities
Derivative Liabilities	52.8	50.2
Embedded Derivative in Modified Coinsurance Arrangement	46.7	87.6
Unfunded commitment to fund Investment Partnerships	5.0	5.0
Fair Value
Assets
Fixed Maturity Securities	44,217.3	43,354.4
Mortgage Loans	2,122.2	2,013.9
Policy Loans	3,564.2	3,498.0
Other Long-term Investments
Derivative Assets	32.7	49.8
Equity Securities	1.2	1.4
Miscellaneous Long-Term Investments	541.9	474.4
Policyholders' Funds
Deferred Annuity Products	597.4	608.8
Supplementary Contracts without Life Contingencies	608.8	641.1
Short-term Debt	0.0	366.2
Long-term Debt	3,175.8	2,645.9
Advances from FHLB	350.0	350.0
Other Liabilities
Derivative Liabilities	52.8	50.2
Embedded Derivative in Modified Coinsurance Arrangement	46.7	87.6
Unfunded commitment to fund Investment Partnerships	$ 5.0	$ 5.0

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