Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 36,640.7	[1]	$ 36,546.6
Gross Unrealized Gain	6,068.2		3,726.4
Gross Unrealized Loss	222.2		237.4
Fixed Maturity Securities	42,486.7		40,035.6
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0		3.9
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,005.8	[1]	981.7
Gross Unrealized Gain	299.7		128.6
Gross Unrealized Loss	1.1		8.6
Fixed Maturity Securities	1,304.4		1,101.7
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			0
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,377.8	[1]	1,271.0
Gross Unrealized Gain	222.6		21.5
Gross Unrealized Loss	8.8		47.3
Fixed Maturity Securities	1,591.6		1,245.2
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			0
Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,139.4	[1]	1,248.6
Gross Unrealized Gain	237.3		160.7
Gross Unrealized Loss	0		0
Fixed Maturity Securities	1,376.7		1,409.3
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			0
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,015.7	[1]	8,874.2
Gross Unrealized Gain	1,646.2		854.3
Gross Unrealized Loss	28.6		44.3
Fixed Maturity Securities	10,633.3		9,684.2
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			0
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,634.6	[1]	3,047.8
Gross Unrealized Gain	344.1		338.3
Gross Unrealized Loss	5.5		0.6
Fixed Maturity Securities	2,973.2		3,385.5
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			0
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	21,411.6	[1]	21,067.5
Gross Unrealized Gain	3,314.8		2,221.3
Gross Unrealized Loss	176.3		134.1
Fixed Maturity Securities	24,550.1		23,154.7
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			3.9	[2]
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	55.8	[1]	55.8
Gross Unrealized Gain	3.5		1.7
Gross Unrealized Loss	1.9		2.5
Fixed Maturity Securities	57.4		55.0
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss			$ 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] Accumulated Other Comprehensive Income (Loss)