Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Assets
Fixed Maturity Securities	$ 44,900.9	$ 42,344.4
Mortgage Loans	1,823.8	1,815.1
Policy Loans	3,222.4	3,276.0
Other Long-term Investments
Equity Securities	15.4	16.4
Liabilities
Long-term Debt	2,792.0	2,612.0
Other Liabilities
Unfunded commitment to fund Investment Partnerships	15.7
Carrying Amount
Assets
Fixed Maturity Securities	44,900.9	42,344.4
Mortgage Loans	1,823.8	1,815.1
Policy Loans	3,222.4	3,276.0
Other Long-term Investments
Derivative Assets	11.4	10.8
Equity Securities	15.4	16.4
Miscellaneous Long-Term Investments	486.5	475.2
Liabilities
Long-term Debt	2,792.0	2,612.0
Policyholders' Funds
Deferred Annuity Products	630.3	631.5
Supplementary Contracts without Life Contingencies	604.3	563.1
Other Liabilities
Derivative Liabilities	133.2	135.6
Embedded Derivative in Modified Coinsurance Arrangement	32.3	53.2
Unfunded commitment to fund Investment Partnerships	15.7	27.2
Fair Value
Assets
Fixed Maturity Securities	44,900.9	42,344.4
Mortgage Loans	2,011.2	1,980.2
Policy Loans	3,311.3	3,339.6
Other Long-term Investments
Derivative Assets	11.4	10.8
Equity Securities	15.4	16.4
Miscellaneous Long-Term Investments	486.5	475.2
Liabilities
Long-term Debt	3,086.5	2,824.4
Policyholders' Funds
Deferred Annuity Products	630.3	631.5
Supplementary Contracts without Life Contingencies	604.3	563.1
Other Liabilities
Derivative Liabilities	133.2	135.6
Embedded Derivative in Modified Coinsurance Arrangement	32.3	53.2
Unfunded commitment to fund Investment Partnerships	$ 15.7	$ 27.2

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