Fair Values of Financial Instruments - Fair Value of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	$ 2,047	$ 2,114
Tax exempt, Fair Value	19,340	18,421
Taxable, Fair Value	19,249	18,670
Total fixed maturities, Fair Value	38,589	37,091
Equity securities	1,888	1,810
Long Term Debt	3,300	3,300
Residential Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	230	299
Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,098	1,365
U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	2,017	802
Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,123	9,314
Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,781	6,890
Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	2,047	2,114
Tax exempt, Fair Value	19,340	18,421
Taxable, Fair Value	19,249	18,670
Total fixed maturities, Fair Value	38,589	37,091
Equity securities	1,888	1,810
Total fixed maturities and equity securities, Fair Value	42,524	41,015
Long Term Debt	3,975	3,806
Fair Value [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	2,017	802
Fair Value [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,123	9,314
Fair Value [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,781	6,890
Residential Mortgage-backed Securities [Member] | Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	230	299
Commercial Mortgage-backed Securities [Member] | Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,098	1,365
Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	193	399
Equity securities	1,882	1,803
Total fixed maturities and equity securities, Fair Value	2,075	2,202
Level 2 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	1,854	1,715
Tax exempt, Fair Value	19,337	18,416
Taxable, Fair Value	19,113	18,500
Total fixed maturities, Fair Value	38,450	36,916
Total fixed maturities and equity securities, Fair Value	40,304	38,631
Long Term Debt	3,975	3,806
Level 2 [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	2,017	802
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,000	9,179
Level 2 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,772	6,881
Level 2 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	226	293
Level 2 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,098	1,345
Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Tax exempt, Fair Value	3	5
Taxable, Fair Value	136	170
Total fixed maturities, Fair Value	139	175
Equity securities	6	7
Total fixed maturities and equity securities, Fair Value	145	182
Level 3 [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	123	135
Level 3 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9	9
Level 3 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	4	6
Level 3 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value		$ 20