SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts [Roll Forward]
Balance at beginning of period	$ 51	$ 55	$ 50
Additions charged to costs and expenses	4	11	18
Other additions	0	0	0
Deductions	4	15	13
Balance at end of period	$ 51	$ 51	$ 55