Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 8,924	$ 8,638
Gross Unrealized Gains	627	558
Gross Unrealized Losses	29	75
Fair Value	9,522	9,121
Debt Securities | States, Municipalities And Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,294	3,107
Gross Unrealized Gains	173	125
Gross Unrealized Losses	3	21
Fair Value	3,464	3,211
Debt Securities | Convertibles And Bonds With Warrants Attached
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	7	17
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	7	17
Debt Securities | United States Government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	7	7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	7	7
Debt Securities | Government-Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	222	227
Gross Unrealized Gains	0	0
Gross Unrealized Losses	12	27
Fair Value	210	200
Debt Securities | Foreign Government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	10	10
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	10	10
Debt Securities | Commercial Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	250	148
Gross Unrealized Gains	5	0
Gross Unrealized Losses	1	5
Fair Value	254	143
Debt Securities | Corporate Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,134	5,122
Gross Unrealized Gains	449	433
Gross Unrealized Losses	13	22
Fair Value	5,570	5,533
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,697	2,523
Gross Unrealized Gains	1,976	1,856
Gross Unrealized Losses	7	4
Fair Value	4,666	4,375
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,552	2,396
Gross Unrealized Gains	1,942	1,818
Gross Unrealized Losses	6	1
Fair Value	4,488	4,213
Equity Securities | Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	145	127
Gross Unrealized Gains	34	38
Gross Unrealized Losses	1	3
Fair Value	178	162
Fixed Maturities And Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	11,621	11,161
Gross Unrealized Gains	2,603	2,414
Gross Unrealized Losses	36	79
Fair Value	$ 14,188	$ 13,496