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, 2013	Dec. 31, 2012
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 8,502	$ 8,222
Gross Unrealized Gains	600	874
Gross Unrealized Losses	64	3
Fair Value	9,038	9,093
Debt Securities | States, Municipalities And Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,046	3,040
Gross Unrealized Gains	146	250
Gross Unrealized Losses	17	1
Fair Value	3,175	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	209	164
Gross Unrealized Gains	0	0
Gross Unrealized Losses	19	0
Fair Value	190	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	79	27
Gross Unrealized Gains	0	1
Gross Unrealized Losses	3	0
Fair Value	76	28
Debt Securities | Corporate Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,135	4,950
Gross Unrealized Gains	454	622
Gross Unrealized Losses	25	2
Fair Value	5,564	5,570
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,471	2,369
Gross Unrealized Gains	1,516	1,014
Gross Unrealized Losses	3	10
Fair Value	3,984	3,373
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,394	2,270
Gross Unrealized Gains	1,482	977
Gross Unrealized Losses	3	9
Fair Value	3,873	3,238
Equity Securities | Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	77	99
Gross Unrealized Gains	34	37
Gross Unrealized Losses	0	1
Fair Value	111	135
Fixed Maturities And Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	10,973	10,591
Gross Unrealized Gains	2,116	1,888
Gross Unrealized Losses	67	13
Fair Value	$ 13,022	$ 12,466