SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Part D Subsidy/Discount Payments, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Part D subsidy/discount payments	$ (14,889)	$ (13,348)	$ (11,762)
Part D subsidy/discount reimbursements	14,628	12,410	11,202
Net payments	$ (261)	$ (938)	$ (560)