Carrying Amount and Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Assets
Fixed Maturity Securities	$ 45,862.0	$ 43,354.4
Mortgage Loans	1,941.3	1,883.6
Policy Loans	3,316.2	3,395.4
Other Long-term Investments
Derivative Assets	40.4	49.8
Equity Securities	1.5	1.4
Policyholders' Funds
Long-term Debt	3,042.6	2,449.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	349.5	350.0
Other Liabilities
Derivative Liabilities	140.0	137.8
Unfunded Commitments to Investment Partnerships	4.8
Carrying Amount
Assets
Fixed Maturity Securities	45,862.0	43,354.4
Mortgage Loans	1,941.3	1,883.6
Policy Loans	3,316.2	3,395.4
Other Long-term Investments
Derivative Assets	40.4	49.8
Equity Securities	1.5	1.4
Miscellaneous Long-Term Investments	499.9	474.4
Policyholders' Funds
Deferred Annuity Products	604.9	608.8
Supplementary Contracts without Life Contingencies	611.9	641.1
Short-term Debt	350.7	352.0
Long-term Debt	3,042.6	2,449.4
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	349.5	350.0
Other Liabilities
Derivative Liabilities	57.0	50.2
Embedded Derivative in Modified Coinsurance Arrangement	83.0	87.6
Unfunded Commitments to Investment Partnerships	5.3	5.0
Fair Value
Assets
Fixed Maturity Securities	45,862.0	43,354.4
Mortgage Loans	2,119.2	2,013.9
Policy Loans	3,442.2	3,498.0
Other Long-term Investments
Derivative Assets	40.4	49.8
Equity Securities	1.5	1.4
Miscellaneous Long-Term Investments	499.9	474.4
Policyholders' Funds
Deferred Annuity Products	604.9	608.8
Supplementary Contracts without Life Contingencies	611.9	641.1
Short-term Debt	354.6	366.2
Long-term Debt	3,333.8	2,645.9
Payables for Collateral on Investments
Federal Home Loan Bank (FHLB) Funding Agreements	349.5	350.0
Other Liabilities
Derivative Liabilities	57.0	50.2
Embedded Derivative in Modified Coinsurance Arrangement	83.0	87.6
Unfunded Commitments to Investment Partnerships	$ 5.3	$ 5.0

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