SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Part D Subsidy/Discount Payments, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accounting Policies [Abstract]
Part D subsidy/discount payments	$ (17,582)	$ (16,530)	$ (14,889)
Part D subsidy/discount reimbursements	18,353	18,498	14,628
Net reimbursements (payments)	$ 771	$ 1,968	$ (261)