BASIS OF PRESENTATION - Cash and Cash Equivalents (Details) - USD ($) $ in Millions |
3 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Cash and Cash Equivalents | ||||
| Cash and cash equivalents | $ 974 | $ 572 | $ 611 | $ 716 |
| Accrued property and equipment purchases for items received but not yet paid | 72 | 117 | ||
| Non-cancellable capital leases primarily for buildings and equipment | 20 | $ 34 | ||
| Captive insurance subsidiaries | ||||
| Cash and Cash Equivalents | ||||
| Cash and cash equivalents | 187 | 179 | ||
| Health plan-related businesses | ||||
| Cash and Cash Equivalents | ||||
| Cash and cash equivalents | 32 | 30 | ||
| Accounts payable | ||||
| Cash and Cash Equivalents | ||||
| Book overdrafts classified as accounts payable | 244 | 311 | ||
| Accrued property and equipment purchases for items received but not yet paid | $ 52 | $ 79 | ||