BASIS OF PRESENTATION AND ACCOUNTING CHANGES - Accounting Changes (Details) - USD ($) $ in Millions		Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Operating lease, right-of-use asset		$ 4,100
Operating lease, liability		$ 4,112
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle
Operating lease, right-of-use asset			$ 4,400
Operating lease, liability			4,400
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect adjustment after tax	[1]			$ 151	$ (84)
Retained earnings | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect adjustment after tax			$ 151

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