INVESTMENT SECURITIES - Schedule of Contractual Maturities of Investment in Available-For-Sale Debt Securities (Excluding Mortgage and Asset-Backed Securities) (Details) $ in Millions	Sep. 30, 2017 USD ($)
Amortized cost
Within one year	$ 5,342
After one year through five years	3,577
After five years through ten years	5,639
After ten years	16,994
Estimated fair value
Within one year	5,344
After one year through five years	3,796
After five years through ten years	6,171
After ten years	$ 20,395