Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	$ 7,213	$ 7,633
Partially-owned insurance companies	449	454
Total assets	7,662	8,087
Short-term debt	1,402	1,401
Long-term debt	4,861	3,916
Trust preferred securities	449	446
Total liabilities	6,712	5,763
Financial Instruments Fair Value [Member] | US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	997	1,083
Financial Instruments Fair Value [Member] | Foreign [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	937	964
Financial Instruments Fair Value [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	2,202	2,275
Financial Instruments Fair Value [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	1,893	2,116
Financial Instruments Fair Value [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	1,184	1,195
Fair Value, Inputs, Level 1 [Member] | Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	641	619
Total assets	641	619
Fair Value, Inputs, Level 1 [Member] | Financial Instruments Fair Value [Member] | US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	641	619
Fair Value, Inputs, Level 2 [Member] | Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	6,555	6,996
Total assets	6,555	6,996
Short-term debt	1,402	1,401
Long-term debt	4,861	3,916
Trust preferred securities	449	446
Total liabilities	6,712	5,763
Fair Value, Inputs, Level 2 [Member] | Financial Instruments Fair Value [Member] | US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	356	464
Fair Value, Inputs, Level 2 [Member] | Financial Instruments Fair Value [Member] | Foreign [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	937	964
Fair Value, Inputs, Level 2 [Member] | Financial Instruments Fair Value [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	2,185	2,257
Fair Value, Inputs, Level 2 [Member] | Financial Instruments Fair Value [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	1,893	2,116
Fair Value, Inputs, Level 2 [Member] | Financial Instruments Fair Value [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	1,184	1,195
Fair Value, Inputs, Level 3 [Member] | Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	17	18
Partially-owned insurance companies	449	454
Total assets	466	472
Fair Value, Inputs, Level 3 [Member] | Financial Instruments Fair Value [Member] | US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	0	0
Fair Value, Inputs, Level 3 [Member] | Financial Instruments Fair Value [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	17	18
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Financial Instruments Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	6,867	7,270
Partially-owned insurance companies	449	454
Total assets	7,316	7,724
Short-term debt	1,402	1,401
Long-term debt	4,307	3,360
Trust preferred securities	309	309
Total liabilities	6,018	5,070
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Financial Instruments Carrying Value [Member] | US Treasury and Government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	962	1,044
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Financial Instruments Carrying Value [Member] | Foreign [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	882	910
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Financial Instruments Carrying Value [Member] | Corporate Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	2,064	2,133
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Financial Instruments Carrying Value [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	1,812	2,028
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Financial Instruments Carrying Value [Member] | US States and Political Subdivisions Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	$ 1,147	$ 1,155