Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets
Fixed Maturity Securities	$ 45,852.7	$ 45,064.9
Mortgage Loans	1,896.5	1,856.6
Policy Loans	3,283.2us-gaap_LoansInsurancePolicy	3,306.6us-gaap_LoansInsurancePolicy
Other Long-term Investments
Derivative Assets	53.2us-gaap_DerivativeAssets	28.0us-gaap_DerivativeAssets
Equity Securities	11.9	12.5
Policyholders' Funds
Long-term Debt	2,603.3	2,628.7
Other Liabilities
Derivative Liabilities	125.6us-gaap_DerivativeLiabilities	142.8us-gaap_DerivativeLiabilities
Federal Home Loan Bank (FHLB) Funding Agreements	26.0
Unfunded Commitment to Fund Investment Partnerships	8.8
Carrying Amount
Assets
Fixed Maturity Securities	45,852.7	45,064.9
Mortgage Loans	1,896.5	1,856.6
Policy Loans	3,283.2	3,306.6
Other Long-term Investments
Derivative Assets	53.2	28.0
Equity Securities	11.9	12.5
Miscellaneous Long-Term Investments	472.2	438.7
Policyholders' Funds
Deferred Annuity Products	618.2	621.4
Supplementary Contracts without Life Contingencies	615.3	600.4
Short-term Debt	151.9	151.9
Long-term Debt	2,603.3	2,628.7
Other Liabilities
Derivative Liabilities	71.8	92.9
Embedded Derivative in Modified Coinsurance Arrangement	53.8	49.9
Federal Home Loan Bank (FHLB) Funding Agreements	26.0	0
Unfunded Commitment to Fund Investment Partnerships	8.8	12.8
Fair Value
Assets
Fixed Maturity Securities	45,852.7	45,064.9
Mortgage Loans	2,080.4	2,024.2
Policy Loans	3,388.6	3,407.6
Other Long-term Investments
Derivative Assets	53.2	28.0
Equity Securities	11.9	12.5
Miscellaneous Long-Term Investments	472.2	438.7
Policyholders' Funds
Deferred Annuity Products	618.2	621.4
Supplementary Contracts without Life Contingencies	615.3	600.4
Short-term Debt	157.0	158.9
Long-term Debt	2,906.8	2,912.6
Other Liabilities
Derivative Liabilities	71.8	92.9
Embedded Derivative in Modified Coinsurance Arrangement	53.8	49.9
Federal Home Loan Bank (FHLB) Funding Agreements	26.0	0
Unfunded Commitment to Fund Investment Partnerships	$ 8.8	$ 12.8

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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