SUPPLEMENTARY INFORMATION - Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Supplemental Cash Flow Elements [Abstract]
Operating revenue	$ 27	$ 25	$ 81	$ 66
Operating income	7	7	23	19
Net income	$ 2	$ 2	$ 9	$ 6

The amount of net income (loss) reported by an equity method investment of the entity.

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