Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 36,746.7	$ 36,546.6
Gross Unrealized Gain	5,957.6	3,726.4
Gross Unrealized Loss	269.6	237.4
Fixed Maturity Securities	42,434.7	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]
Amortized Cost	998.8	981.7
Gross Unrealized Gain	296.0	128.6
Gross Unrealized Loss	1.2	8.6
Fixed Maturity Securities	1,293.6	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]
Amortized Cost	1,360.5	1,271.0
Gross Unrealized Gain	229.0	21.5
Gross Unrealized Loss	3.1	47.3
Fixed Maturity Securities	1,586.4	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]
Amortized Cost	1,254.3	1,248.6
Gross Unrealized Gain	220.4	160.7
Gross Unrealized Loss	0	0
Fixed Maturity Securities	1,474.7	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]
Amortized Cost	8,935.8	8,874.2
Gross Unrealized Gain	1,593.5	854.3
Gross Unrealized Loss	28.6	44.3
Fixed Maturity Securities	10,500.7	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]
Amortized Cost	2,764.4	3,047.8
Gross Unrealized Gain	386.5	338.3
Gross Unrealized Loss	3.9	0.6
Fixed Maturity Securities	3,147.0	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]
Amortized Cost	21,377.1	21,067.5
Gross Unrealized Gain	3,229.8	2,221.3
Gross Unrealized Loss	231.1	134.1
Fixed Maturity Securities	24,375.8	23,154.7
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss		3.9	[1]
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	55.8	55.8
Gross Unrealized Gain	2.4	1.7
Gross Unrealized Loss	1.7	2.5
Fixed Maturity Securities	56.5	55.0
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss		$ 0

[1] Accumulated Other Comprehensive Income (Loss)