MEDICARE PART D (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Other current assets	$ 4,178	$ 2,949
Trade accounts payable and accrued expenses	(5,413)	(4,069)
Other long-term assets	1,380	2,166
Other long-term liabilities	(603)	(237)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	6	4
Trade accounts payable and accrued expenses	(214)	(255)
Net current (liability) asset	(208)	(251)
Other long-term assets	27	0
Other long-term liabilities	(122)	(28)
Net long-term liability	(95)	(28)
Total net asset (liability)	(303)	(279)
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	268	101
Trade accounts payable and accrued expenses	(1,688)	(1,085)
Net current (liability) asset	(1,420)	(984)
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term liability	0	0
Total net asset (liability)	$ (1,420)	$ (984)