Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Assets
Fixed Maturity Securities	$ 43,516.9	$ 42,486.7
Mortgage Loans	1,652.5	1,612.3
Policy Loans	3,048.1	3,051.4
Other Long-term Investments
Derivatives	118.4	137.7
Equity Securities	11.8	11.2
Miscellaneous Long-term Investments	655.0	639.2
Policyholders' Funds
Long-term Debt	2,540.3	2,570.2
Other Liabilities
Derivatives	292.4	309.4
Unfunded commitment to fund Investment Partnerships	122.0
Carrying Amount
Assets
Fixed Maturity Securities	43,516.9	42,486.7
Mortgage Loans	1,652.5	1,612.3
Policy Loans	3,048.1	3,051.4
Other Long-term Investments
Derivatives	118.4	137.7
Equity Securities	11.8	11.2
Miscellaneous Long-term Investments	455.3	436.4
Policyholders' Funds
Deferred Annuity Products	643.8	641.1
Supplementary Contracts without Life Contingencies	514.4	502.6
Long-term Debt	2,540.3	2,570.2
Other Liabilities
Derivatives	165.7	173.7
Embedded Derivative in Modified Coinsurance Arrangement	126.7	135.7
Unfunded commitment to fund Investment Partnerships	122.0	160.6
Fair Value
Assets
Fixed Maturity Securities	43,516.9	42,486.7
Mortgage Loans	1,843.7	1,789.8
Policy Loans	3,128.7	3,124.4
Other Long-term Investments
Derivatives	118.4	137.7
Equity Securities	11.8	11.2
Miscellaneous Long-term Investments	455.3	436.4
Policyholders' Funds
Deferred Annuity Products	643.8	641.1
Supplementary Contracts without Life Contingencies	514.4	502.6
Long-term Debt	2,627.5	2,540.2
Other Liabilities
Derivatives	165.7	173.7
Embedded Derivative in Modified Coinsurance Arrangement	126.7	135.7
Unfunded commitment to fund Investment Partnerships	$ 122.0	$ 160.6

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