Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 36,852.5	$ 36,546.6
Gross Unrealized Gain	4,088.1	3,726.4
Gross Unrealized Loss	172.2	237.4
Fixed Maturity Securities	40,768.4	40,035.6
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss		3.9
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	994.6	981.7
Gross Unrealized Gain	146.6	128.6
Gross Unrealized Loss	10.7	8.6
Fixed Maturity Securities	1,130.5	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	1,334.8	1,271.0
Gross Unrealized Gain	60.4	21.5
Gross Unrealized Loss	26.1	47.3
Fixed Maturity Securities	1,369.1	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	1,280.4	1,248.6
Gross Unrealized Gain	169.9	160.7
Gross Unrealized Loss	0.1	0
Fixed Maturity Securities	1,450.2	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	8,966.3	8,874.2
Gross Unrealized Gain	950.9	854.3
Gross Unrealized Loss	30.8	44.3
Fixed Maturity Securities	9,886.4	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	2,882.0	3,047.8
Gross Unrealized Gain	359.7	338.3
Gross Unrealized Loss	2.5	0.6
Fixed Maturity Securities	3,239.2	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]
Amortized Cost	21,338.6	21,067.5
Gross Unrealized Gain	2,397.8	2,221.3
Gross Unrealized Loss	100.9	134.1
Fixed Maturity Securities	23,635.5	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	2.8	1.7
Gross Unrealized Loss	1.1	2.5
Fixed Maturity Securities	57.5	55.0
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss		$ 0

[1] Accumulated Other Comprehensive Income (Loss)