Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Assets
Fixed Maturity Securities	$ 43,354.4	$ 45,064.9
Mortgage Loans	1,883.6	1,856.6
Policy Loans	3,395.4	3,306.6
Other Long-term Investments
Derivative Assets	49.8	28.0
Equity Securities	1.4	12.5
Policyholders' Funds
Long-term Debt	2,475.1	2,628.7
Advances from FHLB	350.0
Other Liabilities
Derivative Liabilities	137.8	142.8
Carrying Amount
Assets
Fixed Maturity Securities	43,354.4	45,064.9
Mortgage Loans	1,883.6	1,856.6
Policy Loans	3,395.4	3,306.6
Other Long-term Investments
Derivative Assets	49.8	28.0
Equity Securities	1.4	12.5
Miscellaneous Long-Term Investments	474.4	438.7
Policyholders' Funds
Deferred Annuity Products	608.8	621.4
Supplementary Contracts without Life Contingencies	641.1	600.4
Short-term Debt	352.4	151.9
Long-term Debt	2,475.1	2,628.7
Advances from FHLB	350.0	0.0
Other Liabilities
Derivative Liabilities	50.2	92.9
Embedded Derivative in Modified Coinsurance Arrangement	87.6	49.9
Unfunded commitment to fund Investment Partnerships	5.0	12.8
Fair Value
Assets
Fixed Maturity Securities	43,354.4	45,064.9
Mortgage Loans	2,013.9	2,024.2
Policy Loans	3,498.0	3,407.6
Other Long-term Investments
Derivative Assets	49.8	28.0
Equity Securities	1.4	12.5
Miscellaneous Long-Term Investments	474.4	438.7
Policyholders' Funds
Deferred Annuity Products	608.8	621.4
Supplementary Contracts without Life Contingencies	641.1	600.4
Short-term Debt	366.2	158.9
Long-term Debt	2,645.9	2,912.6
Advances from FHLB	350.0	0.0
Other Liabilities
Derivative Liabilities	50.2	92.9
Embedded Derivative in Modified Coinsurance Arrangement	87.6	49.9
Unfunded commitment to fund Investment Partnerships	$ 5.0	$ 12.8

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