Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 8,428	$ 8,222
Gross Unrealized Gains	621	874
Gross Unrealized Losses	57	3
Fair Value	8,992	9,093
Debt Securities | States, Municipalities And Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,044	3,040
Gross Unrealized Gains	162	250
Gross Unrealized Losses	18	1
Fair Value	3,188	3,289
Debt Securities | Convertibles And Bonds With Warrants Attached
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	16	31
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	16	31
Debt Securities | United States Government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	7	7
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	0
Fair Value	7	8
Debt Securities | Government-Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	208	164
Gross Unrealized Gains	0	0
Gross Unrealized Losses	13	0
Fair Value	195	164
Debt Securities | Foreign Government
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	10	3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	10	3
Debt Securities | Commercial Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	63	27
Gross Unrealized Gains	0	1
Gross Unrealized Losses	3	0
Fair Value	60	28
Debt Securities | Corporate Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,080	4,950
Gross Unrealized Gains	459	622
Gross Unrealized Losses	23	2
Fair Value	5,516	5,570
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,452	2,369
Gross Unrealized Gains	1,435	1,014
Gross Unrealized Losses	12	10
Fair Value	3,875	3,373
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,375	2,270
Gross Unrealized Gains	1,398	977
Gross Unrealized Losses	12	9
Fair Value	3,761	3,238
Equity Securities | Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	77	99
Gross Unrealized Gains	37	37
Gross Unrealized Losses	0	1
Fair Value	114	135
Fixed Maturities And Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	10,880	10,591
Gross Unrealized Gains	2,056	1,888
Gross Unrealized Losses	69	13
Fair Value	$ 12,867	$ 12,466