MEDICARE PART D (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Other current assets	$ 2,949	$ 3,438
Trade accounts payable and accrued expenses	(4,069)	(2,467)
Other long-term liabilities	(237)	(263)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	4	8
Trade accounts payable and accrued expenses	(255)	(158)
Net current (liability) asset	(251)	(150)
Other long-term liabilities	(28)	0
Total net (liability) asset	(279)	(150)
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	101	1,001
Trade accounts payable and accrued expenses	(1,085)	(128)
Net current (liability) asset	(984)	873
Other long-term liabilities	0	0
Total net (liability) asset	$ (984)	$ 873