SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Changes (Details) - USD ($) $ in Millions		3 Months Ended									12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016		Jan. 01, 2018	Jan. 01, 2017	Jan. 01, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle
Impact of non-cash charge related to Tax Reform						$ 22,600							$ 22,594
Tax Reform benefit											$ 94
Benefit relating to reduction in valuation allowance against FTC carry-forwards and U.S. residual DTAs		$ 1,200
Charge for quasi-territorial tax system		200
Charge related to impact of deemed repatriation of undistributed earnings		900
Revenues, net of interest expense		17,124	$ 18,389	$ 18,469	$ 18,872	17,504		$ 18,419	$ 18,155	$ 18,366	72,854	[1]	72,444	[1]	$ 70,797	[1]
Operating expenses		9,893	$ 10,311	$ 10,712	$ 10,925	10,332		$ 10,417	$ 10,760	$ 10,723	41,841	[1]	42,232	[1]	42,338	[1]
Retained earnings		151,347				138,425					151,347		138,425
Fair value		288,038				290,725					288,038		290,725
Net cash used in investing activities of continuing operations											(73,118)		(38,751)		(54,135)
Stockholders’ equity		197,074				201,672					197,074		201,672		226,143
Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle
Increase in deferred income tax assets																	$ 300
Retained earnings																	(80)
Prepaid tax																	(380)
Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle
Fair value																	4,000
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle
Retained earnings																			$ (15)
Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle
Retained earnings		3,300									3,300
Accounting Standards Update 2017-08 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle
Stockholders’ equity																		$ (156)
Accounting Standards Update 2016-01 And 2018-03
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle																	3
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle
Net cash used in investing activities of continuing operations													19,300		25,300
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle
Stockholders’ equity		151,347				138,425					151,347		138,425		146,477					$ 133,841
Cumulative effect of new accounting principle	[2]					(84)							(84)		(660)					15
Retained earnings | Accounting Standards Update 2017-08 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle																		(660)
Accumulated other comprehensive income (loss)
New Accounting Pronouncements or Change in Accounting Principle
Stockholders’ equity		$ (37,170)				(34,668)					(37,170)		(34,668)		(32,381)					(29,344)
Cumulative effect of new accounting principle						$ (3)	[2]						(3)	[2]	$ 504	[2]	$ (3)	504	$ (15)	$ (15)	[2]
Accumulated other comprehensive income (loss) | Accounting Standards Update 2017-08 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle																		$ 504
Restatement | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle
Revenues, net of interest expense											1,000		1,000
Operating expenses											$ 1,000		$ 1,000

[1] Certain prior-period revenue and expense lines and totals were reclassified to conform to the current period’s presentation. See Notes 1 and 3 to the Consolidated Financial Statements.

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