Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets
Fixed Maturity Securities	$ 45,064.9	$ 42,344.4
Mortgage Loans	1,856.6	1,815.1
Policy Loans	3,306.6us-gaap_LoansInsurancePolicy	3,276.0us-gaap_LoansInsurancePolicy
Other Long-term Investments
Derivative Assets	28.0us-gaap_DerivativeAssets	10.8us-gaap_DerivativeAssets
Equity Securities	12.5	16.4
Policyholders' Funds
Long-term Debt	2,628.7	2,612.0
Other Liabilities
Derivative Liabilities	142.8us-gaap_DerivativeLiabilities	188.8us-gaap_DerivativeLiabilities
Unfunded commitment to fund Investment Partnerships	12.8
Carrying Amount
Assets
Fixed Maturity Securities	45,064.9	42,344.4
Mortgage Loans	1,856.6	1,815.1
Policy Loans	3,306.6	3,276.0
Other Long-term Investments
Derivative Assets	28.0	10.8
Equity Securities	12.5	16.4
Miscellaneous Long-Term Investments	485.5	475.2
Policyholders' Funds
Deferred Annuity Products	621.4	631.5
Supplementary Contracts without Life Contingencies	600.4	563.1
Short-term Debt	151.9	0
Long-term Debt	2,628.7	2,612.0
Other Liabilities
Derivative Liabilities	92.9	135.6
Embedded Derivative in Modified Coinsurance Arrangement	49.9	53.2
Unfunded commitment to fund Investment Partnerships	12.8	27.2
Fair Value
Assets
Fixed Maturity Securities	45,064.9	42,344.4
Mortgage Loans	2,024.2	1,980.2
Policy Loans	3,407.6	3,339.6
Other Long-term Investments
Derivative Assets	28.0	10.8
Equity Securities	12.5	16.4
Miscellaneous Long-Term Investments	485.5	475.2
Policyholders' Funds
Deferred Annuity Products	621.4	631.5
Supplementary Contracts without Life Contingencies	600.4	563.1
Short-term Debt	158.9	0
Long-term Debt	2,912.6	2,824.4
Other Liabilities
Derivative Liabilities	92.9	135.6
Embedded Derivative in Modified Coinsurance Arrangement	49.9	53.2
Unfunded commitment to fund Investment Partnerships	$ 12.8	$ 27.2

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