Gross Unrealized Losses and Fair Values for Investments in Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 4,157	$ 3,178
Less than 12 Months, Unrealized Loss	(25)	(111)
12 Months or Greater, Fair Value	102	42
12 Months or Greater, Unrealized Loss	(5)	(3)
Total Fair Value	4,259	3,220
Total Unrealized Loss	(30)	(114)
Foreign government bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	509	302
Less than 12 Months, Unrealized Loss	(2)	(11)
12 Months or Greater, Fair Value	12	6
12 Months or Greater, Unrealized Loss	(1)	0
Total Fair Value	521	308
Total Unrealized Loss	(3)	(11)
Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	820	2,160
Less than 12 Months, Unrealized Loss	(10)	(97)
12 Months or Greater, Fair Value	81	17
12 Months or Greater, Unrealized Loss	(4)	(1)
Total Fair Value	901	2,177
Total Unrealized Loss	(14)	(98)
Agency residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	1,300	716
Less than 12 Months, Unrealized Loss	(6)	(3)
12 Months or Greater, Fair Value	0	19
12 Months or Greater, Unrealized Loss	0	(2)
Total Fair Value	1,300	735
Total Unrealized Loss	(6)	(5)
US Government notes
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	842
Less than 12 Months, Unrealized Loss	(1)
12 Months or Greater, Fair Value	0
12 Months or Greater, Unrealized Loss	0
Total Fair Value	842
Total Unrealized Loss	(1)
Municipal Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	686
Less than 12 Months, Unrealized Loss	(6)
12 Months or Greater, Fair Value	9
12 Months or Greater, Unrealized Loss	0
Total Fair Value	695
Total Unrealized Loss	$ (6)