INVESTMENT SECURITIES - Estimated Fair Value and Gross Unrealized Losses (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Estimated fair value
Less than 12 months	$ 9,210	$ 3,093
12 months or more	2,623	4,949
Gross unrealized losses
Less than 12 months	(282)	(23)
12 months or more	(140)	(91)
U.S. corporate
Estimated fair value
Less than 12 months	5,536	502
12 months or more	566	605
Gross unrealized losses
Less than 12 months	(170)	(6)
12 months or more	(65)	(30)
Non-U.S. corporate
Estimated fair value
Less than 12 months	1,043	1,169
12 months or more	1,282	3,685
Gross unrealized losses
Less than 12 months	(33)	(4)
12 months or more	(5)	(10)
State and municipal
Estimated fair value
Less than 12 months	317	48
12 months or more	282	272
Gross unrealized losses
Less than 12 months	(6)	(1)
12 months or more	(50)	(39)
Mortgage and asset-backed
Estimated fair value
Less than 12 months	1,826	979
12 months or more	384	318
Gross unrealized losses
Less than 12 months	(47)	(11)
12 months or more	(19)	(12)
Government and agencies
Estimated fair value
Less than 12 months	488	395
12 months or more	109	69
Gross unrealized losses
Less than 12 months	(26)	(2)
12 months or more	$ (1)	$ 0