MEDICARE PART D (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Other current assets	$ 4,059	$ 2,949
Trade accounts payable and accrued expenses	(6,266)	(4,069)
Other long-term assets	1,805	2,166
Other long-term liabilities	(303)	(237)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	5	4
Trade accounts payable and accrued expenses	(240)	(255)
Net current (liability) asset	(235)	(251)
Other long-term assets	33	0
Other long-term liabilities	(78)	(28)
Net long-term liability	(45)	(28)
Total net (liability) asset	(280)	(279)
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	79	101
Trade accounts payable and accrued expenses	(2,509)	(1,085)
Net current (liability) asset	(2,430)	(984)
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term liability	0	0
Total net (liability) asset	$ (2,430)	$ (984)