Investment Securities (Contractual maturities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014
Amortized Cost
Within one year	$ 1,838
After one year through five years	3,630
After five years through ten years	5,349
After ten years	18,490
Estimated Fair Value
Within one year	1,845
After one year through five years	3,892
After five years through ten years	5,641
After ten years	$ 21,361