BASIS OF PRESENTATION - Cash and Cash Equivalents (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Cash and Cash Equivalents
Cash and cash equivalents	$ 613		$ 262
Accrued property and equipment purchases for items received but not yet paid	65		136
Non-cancellable finance leases entered into	15	$ 36
Non-cancellable operating leases liability entered into	54	$ 28
Captive insurance subsidiaries
Cash and Cash Equivalents
Cash and cash equivalents	152		176
Health plan-related businesses
Cash and Cash Equivalents
Cash and cash equivalents	2		2
Accounts payable
Cash and Cash Equivalents
Book overdrafts classified as accounts payable	225		246
Accrued property and equipment purchases for items received but not yet paid	$ 49		$ 119