Schedule I Summary of Investments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 36,640.7	[1]	$ 36,546.6
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
Derivative Assets	137.7		99.1
Available-for-sale Equity Securities, Amortized Cost Basis	11.1
Equity Securities	11.2		10.4
Other Investments and Securities, at Cost	482.1
Other Long-term Investments	639.2		529.3
Short-term Investments	1,423.5
Investment Owned, at Cost	43,221.1
Investments	49,213.1		46,240.9
US Government and Government Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,005.8	[1]	981.7
Fixed Maturity Securities	1,304.4		1,101.7
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,377.8	[1]	1,271.0
Fixed Maturity Securities	1,591.6		1,245.2
Foreign Government Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,139.4	[1]	1,248.6
Fixed Maturity Securities	1,376.7		1,409.3
Public Utility, Bonds [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,015.7	[1]	8,874.2
Fixed Maturity Securities	10,633.3		9,684.2
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,634.6	[1]	3,047.8
Fixed Maturity Securities	2,973.2		3,385.5
All Other Corporate Bonds [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	21,411.6	[1]	21,067.5
Fixed Maturity Securities	24,550.1		23,154.7
Redeemable Preferred Stock [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	55.8	[1]	55.8
Fixed Maturity Securities	57.4		55.0
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	42,486.7
Mortgage Loans	1,612.3		1,516.8
Policy Loans	3,051.4		2,996.1
Other Long-term Investments
Derivative Assets	137.7	[2]	99.1
Equity Securities	11.2		10.4
Other Long-term Investments	490.3	[3]	419.8
Short-term Investments	1,423.5
Carrying (Reported) Amount, Fair Value Disclosure [Member] | US Government and Government Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	1,304.4
Carrying (Reported) Amount, Fair Value Disclosure [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	1,591.6
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Foreign Government Debt Securities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	1,376.7
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Public Utility, Bonds [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	10,633.3
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	2,973.2
Carrying (Reported) Amount, Fair Value Disclosure [Member] | All Other Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	24,550.1
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Redeemable Preferred Stock [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	$ 57.4

[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.