Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets
Fixed Maturity Securities	$ 43,666.8	$ 42,344.4
Mortgage Loans	1,843.3	1,815.1
Policy Loans	3,268.3	3,276.0
Other Long-term Investments
Equity Securities	15.1	16.4
Policyholders' Funds
Long-term Debt	2,948.2	2,612.0
Other Liabilities
Unfunded commitment to fund Investment Partnerships	17.9
Carrying Amount
Assets
Fixed Maturity Securities	43,666.8	42,344.4
Mortgage Loans	1,843.3	1,815.1
Policy Loans	3,268.3	3,276.0
Other Long-term Investments
Derivative Assets	10.9	10.8
Equity Securities	15.1	16.4
Miscellaneous Long-Term Investments	477.4	475.2
Policyholders' Funds
Deferred Annuity Products	629.7	631.5
Supplementary Contracts without Life Contingencies	585.7	563.1
Long-term Debt	2,948.2	2,612.0
Other Liabilities
Derivative Liabilities	117.7	135.6
Embedded Derivative in Modified Coinsurance Arrangement	44.7	53.2
Unfunded commitment to fund Investment Partnerships	17.9	27.2
Fair Value
Assets
Fixed Maturity Securities	43,666.8	42,344.4
Mortgage Loans	2,009.8	1,980.2
Policy Loans	3,336.3	3,339.6
Other Long-term Investments
Derivative Assets	10.9	10.8
Equity Securities	15.1	16.4
Miscellaneous Long-Term Investments	477.4	475.2
Policyholders' Funds
Deferred Annuity Products	629.7	631.5
Supplementary Contracts without Life Contingencies	585.7	563.1
Long-term Debt	3,194.5	2,824.4
Other Liabilities
Derivative Liabilities	117.7	135.6
Embedded Derivative in Modified Coinsurance Arrangement	44.7	53.2
Unfunded commitment to fund Investment Partnerships	$ 17.9	$ 27.2

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