Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets
Fixed Maturity Securities	$ 44,744.7	$ 42,486.7
Mortgage Loans	1,665.2	1,612.3
Policy Loans	3,117.0	3,051.4
Other Long-term Investments
Derivatives	108.2	137.7
Equity Securities	13.0	11.2
Miscellaneous Long-term Investments	650.9	639.2
Policyholders' Funds
Long-term Debt	2,781.5	2,570.2
Other Liabilities
Derivatives	289.5	309.4
Unfunded commitment to fund Investment Partnerships	112.2
Carrying Amount
Assets
Fixed Maturity Securities	44,744.7	42,486.7
Mortgage Loans	1,665.2	1,612.3
Policy Loans	3,117.0	3,051.4
Other Long-term Investments
Derivatives	108.2	137.7
Equity Securities	13.0	11.2
Miscellaneous Long-term Investments	460.3	436.4
Policyholders' Funds
Deferred Annuity Products	642.2	641.1
Supplementary Contracts without Life Contingencies	536.6	502.6
Long-term Debt	2,781.5	2,570.2
Other Liabilities
Derivatives	182.5	173.7
Embedded Derivative in Modified Coinsurance Arrangement	107.0	135.7
Unfunded commitment to fund Investment Partnerships	112.2	160.6
Fair Value
Assets
Fixed Maturity Securities	44,744.7	42,486.7
Mortgage Loans	1,868.4	1,789.8
Policy Loans	3,199.6	3,124.4
Other Long-term Investments
Derivatives	108.2	137.7
Equity Securities	13.0	11.2
Miscellaneous Long-term Investments	460.3	436.4
Policyholders' Funds
Deferred Annuity Products	642.2	641.1
Supplementary Contracts without Life Contingencies	536.6	502.6
Long-term Debt	2,928.6	2,540.2
Other Liabilities
Derivatives	182.5	173.7
Embedded Derivative in Modified Coinsurance Arrangement	107.0	135.7
Unfunded commitment to fund Investment Partnerships	$ 112.2	$ 160.6

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