Fair Value Measurements (Financial Instruments Not Carried At Fair Value) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	$ 570
Partially-owned insurance companies
Total assets	570
Short-term debt
Long-term debt
Trust preferred securities
Total liabilities
Level 1 [Member] | U.S. Treasury And Agency [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	570
Level 1 [Member] | Foreign [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Level 1 [Member] | Corporate Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Level 1 [Member] | Mortgage-Backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Level 1 [Member] | States, Municipalities, And Political Subdivisions [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	7,762
Partially-owned insurance companies
Total assets	7,762
Short-term debt	1,531
Long-term debt	3,878
Trust preferred securities	404
Total liabilities	5,813
Level 2 [Member] | U.S. Treasury And Agency [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	551
Level 2 [Member] | Foreign [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	955
Level 2 [Member] | Corporate Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	2,338
Level 2 [Member] | Mortgage-Backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	2,784
Level 2 [Member] | States, Municipalities, And Political Subdivisions [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	1,134
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	16
Partially-owned insurance companies	346
Total assets	362
Short-term debt
Long-term debt
Trust preferred securities
Total liabilities
Level 3 [Member] | U.S. Treasury And Agency [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Level 3 [Member] | Foreign [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Level 3 [Member] | Corporate Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	16
Level 3 [Member] | Mortgage-Backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Level 3 [Member] | States, Municipalities, And Political Subdivisions [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity
Total [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	8,348
Partially-owned insurance companies	346
Total assets	8,694
Short-term debt	1,531
Long-term debt	3,878
Trust preferred securities	404
Total liabilities	5,813
Total [Member] | U.S. Treasury And Agency [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	1,121
Total [Member] | Foreign [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	955
Total [Member] | Corporate Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	2,354
Total [Member] | Mortgage-Backed Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	2,784
Total [Member] | States, Municipalities, And Political Subdivisions [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total fixed maturities held to maturity	$ 1,134