Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Assets
Fixed Maturity Securities	$ 44,699.2	$ 43,354.4
Mortgage Loans	1,881.3	1,883.6
Policy Loans	3,343.9	3,395.4
Other Long-term Investments
Derivative Assets	40.1	49.8
Equity Securities	1.5	1.4
Policyholders' Funds
Long-term Debt	2,442.5	2,449.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	151.2	137.8
Unfunded Commitments to Investment Partnerships	4.8
Carrying Amount
Assets
Fixed Maturity Securities	44,699.2	43,354.4
Mortgage Loans	1,881.3	1,883.6
Policy Loans	3,343.9	3,395.4
Other Long-term Investments
Derivative Assets	40.1	49.8
Equity Securities	1.5	1.4
Miscellaneous Long-Term Investments	473.5	474.4
Policyholders' Funds
Deferred Annuity Products	607.0	608.8
Supplementary Contracts without Life Contingencies	632.6	641.1
Short-term Debt	351.4	352.0
Long-term Debt	2,442.5	2,449.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	58.0	50.2
Embedded Derivative in Modified Coinsurance Arrangement	93.2	87.6
Unfunded Commitments to Investment Partnerships	4.8	5.0
Fair Value
Assets
Fixed Maturity Securities	44,699.2	43,354.4
Mortgage Loans	2,042.6	2,013.9
Policy Loans	3,458.0	3,498.0
Other Long-term Investments
Derivative Assets	40.1	49.8
Equity Securities	1.5	1.4
Miscellaneous Long-Term Investments	473.5	474.4
Policyholders' Funds
Deferred Annuity Products	607.0	608.8
Supplementary Contracts without Life Contingencies	632.6	641.1
Short-term Debt	361.7	366.2
Long-term Debt	2,634.4	2,645.9
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	350.0	350.0
Other Liabilities
Derivative Liabilities	58.0	50.2
Embedded Derivative in Modified Coinsurance Arrangement	93.2	87.6
Unfunded Commitments to Investment Partnerships	$ 4.8	$ 5.0

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