Balance Sheet Amounts Associated With Medicare Part D (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Other current assets	$ 5,214	$ 4,007
Trade accounts payable and accrued expenses	(3,554)	(2,095)
Other long-term assets	1,761	1,583
Other long-term liabilities	(281)	(236)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	116	105
Trade accounts payable and accrued expenses	(21)	(36)
Net current asset	95	69
Other long-term assets	94	0
Other long-term liabilities	(16)	0
Net long-term (liability) asset	78	0
Total net asset	173	69
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	1,960	1,690
Trade accounts payable and accrued expenses	(430)	(32)
Net current asset	1,530	1,658
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term (liability) asset	0	0
Total net asset	$ 1,530	$ 1,658