Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets
Mortgage Loans	$ 1,730.4	$ 1,712.7
Policy Loans	3,141.4	3,133.8
Other Long Term Investments
Derivatives	62.1	81.6
Equity Securities	14.4	14.6
Policyholders' Funds
Long-term Debt	2,673.9	2,755.4
Other Liabilities
Derivatives	234.9	254.4
Unfunded commitment to fund Investment Partnerships	75.7
Carrying Amount
Assets
Fixed Maturity Securities	44,436.5	44,973.0
Mortgage Loans	1,730.4	1,712.7
Policy Loans	3,141.4	3,133.8
Other Long Term Investments
Derivatives	62.1	81.6
Equity Securities	14.4	14.6
Miscellaneous Long-Term Investments	438.4	455.1
Policyholders' Funds
Deferred Annuity Products	637.5	640.1
Supplementary Contracts without Life Contingencies	548.3	535.5
Long-term Debt	2,673.9	2,755.4
Other Liabilities
Derivatives	157.4	170.5
Embedded Derivative in Modified Coinsurance Arrangement	77.5	83.9
Unfunded commitment to fund Investment Partnerships	75.7	83.7
Fair Value
Assets
Fixed Maturity Securities	44,436.5	44,973.0
Mortgage Loans	1,959.8	1,937.1
Policy Loans	3,219.7	3,215.3
Other Long Term Investments
Derivatives	62.1	81.6
Equity Securities	14.4	14.6
Miscellaneous Long-Term Investments	438.4	455.1
Policyholders' Funds
Deferred Annuity Products	637.5	640.1
Supplementary Contracts without Life Contingencies	548.3	535.5
Long-term Debt	2,954.9	2,968.8
Other Liabilities
Derivatives	157.4	170.5
Embedded Derivative in Modified Coinsurance Arrangement	77.5	83.9
Unfunded commitment to fund Investment Partnerships	$ 75.7	$ 83.7

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