Carrying Amount and Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Assets
Fixed Maturity Securities	$ 42,486.7		$ 40,035.6
Mortgage Loans	1,612.3	[1]	1,516.8
Policy Loans	3,051.4		2,996.1
Other Long-term Investments
Derivatives	137.7		99.1
Equity Securities	11.2		10.4
Miscellaneous Long-term Investments	639.2		529.3
Policyholders' Funds
Short-term Debt	312.3		225.1
Long-term Debt	2,570.2		2,631.3
Other Liabilities
Derivatives	309.4		295.9
Unfunded commitment to fund Investment Partnerships	160.6
Carrying Amount
Assets
Fixed Maturity Securities	42,486.7		40,035.6
Mortgage Loans	1,612.3		1,516.8
Policy Loans	3,051.4		2,996.1
Other Long-term Investments
Derivatives	137.7	[2]	99.1
Equity Securities	11.2		10.4
Miscellaneous Long-term Investments	490.3	[3]	419.8
Policyholders' Funds
Deferred Annuity Products	641.1		656.3
Supplementary Contracts without Life Contingencies	502.6		508.5
Short-term Debt	312.3		225.1
Long-term Debt	2,570.2		2,631.3
Other Liabilities
Derivatives	173.7		199.6
Embedded Derivative in Modified Coinsurance Arrangement	135.7		96.3
Unfunded commitment to fund Investment Partnerships	160.6		169.9
Fair Value
Assets
Fixed Maturity Securities	42,486.7		40,035.6
Mortgage Loans	1,789.8		1,685.4
Policy Loans	3,124.4		3,044.4
Other Long-term Investments
Derivatives	137.7		99.1
Equity Securities	11.2		10.4
Miscellaneous Long-term Investments	490.3		419.8
Policyholders' Funds
Deferred Annuity Products	641.1		656.3
Supplementary Contracts without Life Contingencies	502.6		508.5
Short-term Debt	312.3		226.8
Long-term Debt	2,540.2		2,483.8
Other Liabilities
Derivatives	173.7		199.6
Embedded Derivative in Modified Coinsurance Arrangement	135.7		96.3
Unfunded commitment to fund Investment Partnerships	$ 160.6		$ 169.9

[1] Amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and accretion of discounts.

[2] Derivatives are carried at fair value.

[3] Difference between cost and carrying value primarily results from changes in our ownership equity since acquisition.