Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Assets
Fixed Maturity Securities	$ 42,411.2	$ 42,486.7
Mortgage Loans	1,615.9	1,612.3
Policy Loans	3,049.0	3,051.4
Other Long-term Investments
Derivatives	111.9	137.7
Equity Securities	12.6	11.2
Miscellaneous Long-term Investments	635.5	639.2
Policyholders' Funds
Long-term Debt	2,552.8	2,570.2
Other Liabilities
Derivatives	297.7	309.4
Unfunded commitment to fund Investment Partnerships	152.0
Carrying Amount
Assets
Fixed Maturity Securities	42,411.2	42,486.7
Mortgage Loans	1,615.9	1,612.3
Policy Loans	3,049.0	3,051.4
Other Long-term Investments
Derivatives	111.9	137.7
Equity Securities	12.6	11.2
Miscellaneous Long-term Investments	452.8	436.4
Policyholders' Funds
Deferred Annuity Products	641.1	641.1
Supplementary Contracts without Life Contingencies	508.7	502.6
Long-term Debt	2,552.8	2,570.2
Other Liabilities
Derivatives	174.3	173.7
Embedded Derivative in Modified Coinsurance Arrangement	123.4	135.7
Unfunded commitment to fund Investment Partnerships	152.0	160.6
Fair Value
Assets
Fixed Maturity Securities	42,411.2	42,486.7
Mortgage Loans	1,792.1	1,789.8
Policy Loans	3,115.4	3,124.4
Other Long-term Investments
Derivatives	111.9	137.7
Equity Securities	12.6	11.2
Miscellaneous Long-term Investments	452.8	436.4
Policyholders' Funds
Deferred Annuity Products	641.1	641.1
Supplementary Contracts without Life Contingencies	508.7	502.6
Long-term Debt	2,585.8	2,540.2
Other Liabilities
Derivatives	174.3	173.7
Embedded Derivative in Modified Coinsurance Arrangement	123.4	135.7
Unfunded commitment to fund Investment Partnerships	$ 152.0	$ 160.6