EMPLOYEE BENEFIT PLANS - PILOT LONG-TERM DISABILITY BENEFITS (Details) - Postretirement Health Coverage [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total liability net of a prefunded trust account	$ 28	$ 25
Prefunded trust account	$ 3	$ 3

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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