BASIS OF PRESENTATION AND SIGNIFICANT EVENTS - Workforce Optimization, Aetna Merger, and Revenue Recognition (Details) $ / shares in Units, $ in Millions		3 Months Ended		12 Months Ended
	Feb. 16, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) employee $ / shares
Business Acquisition [Line Items]
Number of associates impacted | employee				3,600
Percentage of workforce associated impacted				7.80%
Restructuring charges				$ 148
Restructuring charges per diluted share (in dollars per share) | $ / shares				$ 0.64
Restructuring liability, current		$ 94		$ 140
Break up fee received	$ 1,000	0	$ (947)
Accounts receivable		1,276		$ 854
Services Accounts Receivable
Business Acquisition [Line Items]
Accounts receivable		$ 162