Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets
Fixed Maturity Securities	$ 44,715.2	$ 42,344.4
Mortgage Loans	1,807.5	1,815.1
Policy Loans	3,355.5	3,276.0
Other Long-term Investments
Equity Securities	14.7	16.4
Liabilities
Long-term Debt	2,783.3	2,612.0
Other Liabilities
Unfunded commitment to fund Investment Partnerships	13.7
Carrying Amount
Assets
Fixed Maturity Securities	44,715.2	42,344.4
Mortgage Loans	1,807.5	1,815.1
Policy Loans	3,355.5	3,276.0
Other Long-term Investments
Derivative Assets	15.1	10.8
Equity Securities	14.7	16.4
Miscellaneous Long-Term Investments	471.4	475.2
Liabilities
Long-term Debt	2,783.3	2,612.0
Policyholders' Funds
Deferred Annuity Products	626.6	631.5
Supplementary Contracts without Life Contingencies	594.8	563.1
Other Liabilities
Derivative Liabilities	107.2	135.6
Embedded Derivative in Modified Coinsurance Arrangement	34.6	53.2
Unfunded commitment to fund Investment Partnerships	13.7	27.2
Fair Value
Assets
Fixed Maturity Securities	44,715.2	42,344.4
Mortgage Loans	1,981.0	1,980.2
Policy Loans	3,447.0	3,339.6
Other Long-term Investments
Derivative Assets	15.1	10.8
Equity Securities	14.7	16.4
Miscellaneous Long-Term Investments	471.4	475.2
Liabilities
Long-term Debt	3,057.7	2,824.4
Policyholders' Funds
Deferred Annuity Products	626.6	631.5
Supplementary Contracts without Life Contingencies	594.8	563.1
Other Liabilities
Derivative Liabilities	107.2	135.6
Embedded Derivative in Modified Coinsurance Arrangement	34.6	53.2
Unfunded commitment to fund Investment Partnerships	$ 13.7	$ 27.2

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