Investment Securities (Contractual maturities) (Details) $ in Millions	Sep. 30, 2016 USD ($)
Amortized cost
Within one year	$ 11,653
After one year through five years	3,984
After five years through ten years	4,605
After ten years	17,600
Estimated fair value
Within one year	11,656
After one year through five years	4,247
After five years through ten years	5,168
After ten years	$ 22,007