Investment Securities (Details 2) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013 securities	Dec. 31, 2012 securities
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]
Number of securities, less than 12 months	195,000,000	46,000,000
Number of securities, 12 months or more	8,000,000	4,000,000
Number of securities, total	203,000,000	50,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	$ 1,347	$ 100
Estimated Fair Value, 12 months or more	85	73
Estimated Fair Value, Total	1,432	173
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(59)	(1)
Gross Unrealized Losses, 12 months or more	(11)	(4)
Gross Unrealized Losses, Total	(70)	(5)
Ratio Of Fair Value To Amortized Cost Between Ninety And One Hundred Percent [Member]
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]
Number of securities, less than 12 months	180,000,000	46,000,000
Number of securities, 12 months or more	4,000,000	4,000,000
Number of securities, total	184,000,000	50,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	1,231	100
Estimated Fair Value, 12 months or more	27	73
Estimated Fair Value, Total	1,258	173
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(42)	(1)
Gross Unrealized Losses, 12 months or more	(2)	(4)
Gross Unrealized Losses, Total	(44)	(5)
Ratio Of Fair Value To Amortized Cost Less Than Ninety Percent [Member]
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]
Number of securities, less than 12 months	15,000,000
Number of securities, 12 months or more	4,000,000
Number of securities, total	19,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Estimated Fair Value, Less than 12 months	116
Estimated Fair Value, 12 months or more	58
Estimated Fair Value, Total	174
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Gross Unrealized Losses, Less than 12 months	(17)
Gross Unrealized Losses, 12 months or more	(9)
Gross Unrealized Losses, Total	$ (26)