SUPPLEMENTARY INFORMATION - Geographical Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue	$ 1,904	$ 1,870	$ 1,868
Total Assets:	3,706	3,715	3,843
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue	1,832	1,800	1,785
Total Assets:	3,305	3,466	3,635
Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue	72	70	83
Total Assets:	$ 401	$ 249	$ 208