BASIS OF PRESENTATION - Cash and Cash Equivalents (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents
Cash and cash equivalents	$ 403	$ 475	$ 611	$ 716
Accrued property and equipment purchases for items received but not yet paid	76		117
Non-cancellable capital leases primarily for buildings and equipment	50	$ 43
Captive insurance subsidiaries
Cash and Cash Equivalents
Cash and cash equivalents	175		179
Health plan-related businesses
Cash and Cash Equivalents
Cash and cash equivalents	43		30
Accounts payable
Cash and Cash Equivalents
Book overdrafts classified as accounts payable	256		311
Accrued property and equipment purchases for items received but not yet paid	$ 55		$ 79