Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	$ 7,589	$ 6,263
Partially-owned insurance companies	504	470
Total assets	8,093	6,733
Short-term debt	2,571	1,913
Long-term debt	3,690	4,088
Trust preferred securities	462	438
Total liabilities	6,723	6,439
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	850	832
Financial Instruments Fair Value [Member] | Foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	963	897
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	2,423	2,005
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	2,039	1,379
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	1,314	1,150
Level 2 | Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	6,915	5,652
Total assets	6,915	5,652
Short-term debt	2,571	1,913
Long-term debt	3,690	4,088
Trust preferred securities	462	438
Total liabilities	6,723	6,439
Level 2 | 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	191	236
Level 2 | Financial Instruments Fair Value [Member] | Foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	963	897
Level 2 | 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	2,408	1,990
Level 2 | 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	2,039	1,379
Level 2 | 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	1,314	1,150
Level 3 | Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	15	15
Partially-owned insurance companies	504	470
Total assets	519	485
Short-term debt		0
Long-term debt		0
Trust preferred securities		0
Total liabilities		0
Level 3 | 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
Level 3 | Financial Instruments Fair Value [Member] | Foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity		0
Level 3 | 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	15	15
Level 3 | 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		0
Level 3 | 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		0
Cost | Financial Instruments Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	7,331	6,098
Partially-owned insurance companies	504	470
Total assets	7,835	6,568
Short-term debt	2,552	1,901
Long-term debt	3,357	3,807
Trust preferred securities	309	309
Total liabilities	6,218	6,017
Cost | 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	832	820
Cost | Financial Instruments Carrying Value [Member] | Foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	916	864
Cost | 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	2,323	1,922
Cost | 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	1,983	1,341
Cost | 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	1,277	1,151
Level 1 | Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity	659	596
Total assets	659	596
Short-term debt		0
Long-term debt		0
Trust preferred securities		0
Total liabilities		0
Level 1 | 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	659	596
Level 1 | Financial Instruments Fair Value [Member] | Foreign
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Fixed maturities held to maturity		0
Level 1 | 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		0
Level 1 | 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		0
Level 1 | 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		$ 0