Balance Sheet Amounts Associated With Medicare Part D (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Other current assets	$ 4,712	$ 4,007
Trade accounts payable and accrued expenses	(2,212)	(2,095)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	25	105
Trade accounts payable and accrued expenses	(47)	(36)
Total net (liability) asset	(22)	69
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	2,082	1,690
Trade accounts payable and accrued expenses	(63)	(32)
Total net (liability) asset	$ 2,019	$ 1,658