Investments - Securities Available-For-Sale in Gross Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Corporate and other bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	$ 951	$ 846
Gross Unrealized Losses, Less than 12 months	13	13
Estimated Fair Value, 12 Months or Longer	50	108
Gross Unrealized Losses, 12 Months or Longer	5	8
Total Estimated Fair Value	1,001	954
Total Gross Unrealized Losses	18	21
States, municipalities and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	683	254
Gross Unrealized Losses, Less than 12 months	16	5
Estimated Fair Value, 12 Months or Longer	121	165
Gross Unrealized Losses, 12 Months or Longer	43	39
Total Estimated Fair Value	804	419
Total Gross Unrealized Losses	59	44
Residential mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	920	583
Gross Unrealized Losses, Less than 12 months	19	5
Estimated Fair Value, 12 Months or Longer	347	452
Gross Unrealized Losses, 12 Months or Longer	51	66
Total Estimated Fair Value	1,267	1,035
Total Gross Unrealized Losses	70	71
Commercial mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	155	85
Gross Unrealized Losses, Less than 12 months	2	2
Estimated Fair Value, 12 Months or Longer	141	141
Gross Unrealized Losses, 12 Months or Longer	8	15
Total Estimated Fair Value	296	226
Total Gross Unrealized Losses	10	17
Total asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	1,075	668
Gross Unrealized Losses, Less than 12 months	21	7
Estimated Fair Value, 12 Months or Longer	488	593
Gross Unrealized Losses, 12 Months or Longer	59	81
Total Estimated Fair Value	1,563	1,261
Total Gross Unrealized Losses	80	88
U.S. Treasury and obligations of government - sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months		23
Gross Unrealized Losses, Less than 12 months		1
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Total Estimated Fair Value		23
Total Gross Unrealized Losses		1
Redeemable preferred stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	34	28
Gross Unrealized Losses, Less than 12 months	1	1
Estimated Fair Value, 12 Months or Longer
Gross Unrealized Losses, 12 Months or Longer
Total Estimated Fair Value	34	28
Total Gross Unrealized Losses	1	1
Fixed maturity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 months	2,743	1,819
Gross Unrealized Losses, Less than 12 months	51	27
Estimated Fair Value, 12 Months or Longer	659	866
Gross Unrealized Losses, 12 Months or Longer	107	128
Total Estimated Fair Value	3,402	2,685
Total Gross Unrealized Losses	$ 158	$ 155