Investment Securities (Contractual maturities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Amortized Cost
Within one year	$ 4,719
After one year through five years	3,694
After five years through ten years	5,146
After ten years	18,653
Estimated Fair Value
Within one year	4,732
After one year through five years	3,955
After five years through ten years	5,634
After ten years	$ 23,181