INVESTMENT SECURITIES - Contractual Maturities (Details) $ in Millions	Mar. 31, 2020 USD ($)
Amortized cost
Within one year	$ 610
After one year through five years	2,328
After five years through ten years	6,616
After ten years	20,128
Estimated fair value
Within one year	621
After one year through five years	2,400
After five years through ten years	7,226
After ten years	$ 23,603