MEDICARE PART D (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Other current assets	$ 3,564	$ 2,949
Trade accounts payable and accrued expenses	(3,067)	(4,069)
Other long-term assets	1,375	2,166
Other long-term liabilities	(581)	(237)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	15	4
Trade accounts payable and accrued expenses	(103)	(255)
Net current liability	(88)	(251)
Other long-term assets	7	0
Other long-term liabilities	(89)	(28)
Net long-term liability	(82)	(28)
Total net liability	(170)	(279)
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	172	101
Trade accounts payable and accrued expenses	(503)	(1,085)
Net current liability	(331)	(984)
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term liability	0	0
Total net liability	$ (331)	$ (984)