BASIS OF PRESENTATION AND ACCOUNTING CHANGES - Accounting Changes (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Jan. 01, 2018	Jan. 01, 2017	Dec. 31, 2016	Jan. 01, 2016
New Accounting Pronouncements or Change in Accounting Principle
Revenue		$ 18,872	$ 18,366
Operating expenses		10,925	10,723
Decrease in retained earnings		(141,863)		$ (138,425)
Available for sale debt securities		291,523		290,725
Held-to-maturity Securities		52,492		53,320
Reclassification from AOCI		34,619		34,668
Net cash used in investing activities of continuing operations		(32,124)	3,125
Net reduction to total stockholders' equity		(202,866)	(228,997)	(201,672)
Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle
Increase in DTAs					$ 300
Decrease in retained earnings					80
Decrease in prepaid taxes					380
Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle
Available for sale debt securities					4,000
Held-to-maturity Securities					(4,000)
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle
Decrease in retained earnings								$ (15)
Reclassification from AOCI								$ 15
Accounting Standards Update 2016-01 And 2018-03
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect adjustment after tax					$ 3
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle
Net cash used in investing activities of continuing operations			20,300
Early adoption | Accounting Standards Update 2017-08
New Accounting Pronouncements or Change in Accounting Principle
Net reduction to total stockholders' equity						$ 156
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect adjustment after tax	[1]			(84)			$ (660)
Net reduction to total stockholders' equity		(141,863)	(149,071)	(138,425)			(146,477)
Retained earnings | Early adoption | Accounting Standards Update 2017-08
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect adjustment after tax						(660)
Accumulated other comprehensive income (loss)
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect adjustment after tax	[1]			(3)			504
Net reduction to total stockholders' equity		34,619	30,413	34,668			$ 32,381
Accumulated other comprehensive income (loss) | Early adoption | Accounting Standards Update 2017-08
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect adjustment after tax						$ 504
Cash and due from banks
New Accounting Pronouncements or Change in Accounting Principle
Restricted cash		84		39
Deposits with banks
New Accounting Pronouncements or Change in Accounting Principle
Restricted cash		29,000		36,700
Restatement Adjustment | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle
Revenue		250	250	1,000
Operating expenses		$ 250	$ 250	$ 1,000

[1] See Note 1 to the Consolidated Financial Statements for additional details.