BENEFITS PAYABLE - Favorable Medical Claims Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	$ (503)	$ (483)	$ (582)
Retail
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	(398)	(386)	(429)
Group and Specialty
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	(46)	(40)	(46)
Individual Commercial
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	(56)	(56)	(106)
Operating Segments | Retail
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	(398)	(386)	(429)
Operating Segments | Group and Specialty
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	(46)	(40)	(46)
Operating Segments | Individual Commercial
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	(57)	(56)	(106)
Other Businesses
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Favorable Medical Claims Reserve	$ (2)	$ (1)	$ (1)