INVESTMENT SECURITIES - Contractual Maturities (Details) $ in Millions	Jun. 30, 2018 USD ($)
Amortized cost
Within one year	$ 1,903
After one year through five years	3,098
After five years through ten years	6,268
After ten years	18,900
Estimated fair value
Within one year	1,910
After one year through five years	3,192
After five years through ten years	6,673
After ten years	$ 21,105