Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	$ 8,961	$ 8,871
Gross Unrealized Gains	648	607
Gross Unrealized Losses	13	18
Fair Value	9,596	9,460
Debt Securities | Corporate
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,152	5,117
Gross Unrealized Gains	449	420
Gross Unrealized Losses	8	11
Fair Value	5,593	5,526
Debt Securities | States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	3,289	3,267
Gross Unrealized Gains	183	178
Gross Unrealized Losses	2	2
Fair Value	3,470	3,443
Debt Securities | Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	269	250
Gross Unrealized Gains	16	9
Gross Unrealized Losses	0	0
Fair Value	285	259
Debt Securities | Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	227	213
Gross Unrealized Gains	0	0
Gross Unrealized Losses	3	5
Fair Value	224	208
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 | Convertibles and bonds with warrants attached
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 | 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
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,748	2,728
Gross Unrealized Gains	2,052	2,134
Gross Unrealized Losses	11	4
Fair Value	4,789	4,858
Equity Securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	2,595	2,583
Gross Unrealized Gains	2,014	2,099
Gross Unrealized Losses	11	3
Fair Value	4,598	4,679
Equity Securities | Nonredeemable preferred equities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	153	145
Gross Unrealized Gains	38	35
Gross Unrealized Losses	0	1
Fair Value	191	179
Fixed Maturities And Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	11,709	11,599
Gross Unrealized Gains	2,700	2,741
Gross Unrealized Losses	24	22
Fair Value	$ 14,385	$ 14,318