Investment Securities (Contractual maturities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Amortized Cost [Member]
Investment [Line Items]
Within one year	$ 2,957
After one year through five years	5,962
After five years through ten years	5,271
After ten years	17,832
Estimated Fair Value [Member]
Investment [Line Items]
Within one year	2,972
After one year through five years	6,212
After five years through ten years	5,762
After ten years	$ 21,577