Schedule I Summary of Investments (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 37,751.5	[1]	$ 36,640.7
Fixed Maturity Securities	44,973.0		42,486.7
Mortgage Loans	1,712.7	[1]	1,612.3
Policy Loans	3,133.8		3,051.4
Short-term Investments	1,460.3
Investment Owned, at Cost	44,585.0
Investments	51,904.8		49,213.1
Other Long-term Investments
Derivative Assets	81.6		137.7
Available-for-sale Equity Securities, Amortized Cost Basis	11.2
Equity Securities	14.6		11.2
Other Investments and Securities, at Cost	515.5
US Government and Government Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,020.9	[1]	1,005.8
Fixed Maturity Securities	1,348.8		1,304.4
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,498.4	[1]	1,377.8
Fixed Maturity Securities	1,806.8		1,591.6
Foreign Government Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,280.4	[1]	1,139.4
Fixed Maturity Securities	1,507.0		1,376.7
Public Utility, Bonds [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,294.3	[1]	9,015.7
Fixed Maturity Securities	11,144.2		10,633.3
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,927.9	[1]	2,634.6
Fixed Maturity Securities	2,216.5		2,973.2
All Other Corporate Bonds [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	22,696.6	[1]	21,411.6
Fixed Maturity Securities	26,910.4		24,550.1
Redeemable Preferred Stock [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis	33.0	[1]	55.8
Fixed Maturity Securities	39.3		57.4
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	44,973.0
Mortgage Loans	1,712.7		1,612.3
Policy Loans	3,133.8		3,051.4
Short-term Investments	1,460.3
Other Long-term Investments
Derivative Assets	81.6	[2]	137.7
Equity Securities	14.6		11.2
Other Long-term Investments	528.8	[3]
Carrying (Reported) Amount, Fair Value Disclosure [Member] | US Government and Government Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	1,348.8
Carrying (Reported) Amount, Fair Value Disclosure [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	1,806.8
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Foreign Government Debt Securities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	1,507.0
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Public Utility, Bonds [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	11,144.2
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	2,216.5
Carrying (Reported) Amount, Fair Value Disclosure [Member] | All Other Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	26,910.4
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Redeemable Preferred Stock [Member]
Schedule of Investments [Line Items]
Fixed Maturity Securities	39.3
Measurement at Cost [Member]
Other Long-term Investments
Derivative Assets	$ 0

[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 amortized cost and carrying value primarily results from changes in our partnership owner's equity since acquisition.