Balance Sheet Amounts Associated With Medicare Part D (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Other current assets	$ 5,555	$ 4,007
Trade accounts payable and accrued expenses	(3,292)	(2,095)
Other long-term assets	2,005	1,583
Other long-term liabilities	(357)	(236)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	105	105
Trade accounts payable and accrued expenses	(23)	(36)
Net current (liability) asset	82	69
Other long-term assets	121	0
Other long-term liabilities	(18)	0
Net long-term (liability) asset	103	0
Total net asset	185	69
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	2,104	1,690
Trade accounts payable and accrued expenses	(176)	(32)
Net current (liability) asset	1,928	1,658
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term (liability) asset	0	0
Total net asset	$ 1,928	$ 1,658