MEDICARE PART D (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Other current assets	$ 4,410	$ 2,949
Trade accounts payable and accrued expenses	(6,952)	(4,069)
Other long-term assets	1,506	2,166
Other long-term liabilities	(321)	(237)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	6	4
Trade accounts payable and accrued expenses	(232)	(255)
Net current (liability) asset	(226)	(251)
Other long-term assets	64	0
Other long-term liabilities	(87)	(28)
Net long-term liability	(23)	(28)
Total net (liability) asset	(249)	(279)
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	42	101
Trade accounts payable and accrued expenses	(2,588)	(1,085)
Net current (liability) asset	(2,546)	(984)
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term liability	0	0
Total net (liability) asset	$ (2,546)	$ (984)