Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets
Mortgage Loans	$ 1,776.1	$ 1,712.7
Policy Loans	3,242.5	3,133.8
Other Long Term Investments
Equity Securities	15.5	14.6
Derivatives	24.5	81.6
Policyholders' Funds
Long-term Debt	2,631.3	2,755.4
Other Liabilities
Derivatives	208.3	254.4
Unfunded commitment to fund Investment Partnerships	41.9
Carrying Amount
Assets
Fixed Maturity Securities	42,597.6	44,973.0
Mortgage Loans	1,776.1	1,712.7
Policy Loans	3,242.5	3,133.8
Other Long Term Investments
Equity Securities	15.5	14.6
Miscellaneous Long-Term Investments	465.4	455.1
Derivatives	24.5	81.6
Policyholders' Funds
Deferred Annuity Products	634.2	640.1
Supplementary Contracts without Life Contingencies	554.2	535.5
Long-term Debt	2,631.3	2,755.4
Other Liabilities
Derivatives	142.9	170.5
Embedded Derivative in Modified Coinsurance Arrangement	65.4	83.9
Unfunded commitment to fund Investment Partnerships	41.9	83.7
Fair Value
Assets
Fixed Maturity Securities	42,597.6	44,973.0
Mortgage Loans	1,967.8	1,937.1
Policy Loans	3,312.7	3,215.3
Other Long Term Investments
Equity Securities	15.5	14.6
Miscellaneous Long-Term Investments	465.4	455.1
Derivatives	24.5	81.6
Policyholders' Funds
Deferred Annuity Products	634.2	640.1
Supplementary Contracts without Life Contingencies	554.2	535.5
Long-term Debt	2,827.5	2,968.8
Other Liabilities
Derivatives	142.9	170.5
Embedded Derivative in Modified Coinsurance Arrangement	65.4	83.9
Unfunded commitment to fund Investment Partnerships	$ 41.9	$ 83.7

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