Balance Sheet Amounts Associated With Medicare Part D (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Other current assets	$ 1,960	$ 1,670
Trade accounts payable and accrued expenses	(2,174)	(2,042)
Other long-term assets	1,679	1,685
Other long-term liabilities	(327)	(288)
Risk Corridor Settlements
Segment Reporting Information [Line Items]
Other current assets	37	37
Trade accounts payable and accrued expenses	(217)	(393)
Net current (liability) asset	(180)	(356)
Other long-term assets	39	0
Other long-term liabilities	(29)	0
Net long-term asset	10	0
Total net (liability) asset	(170)	(356)
CMS Subsidies/Discounts
Segment Reporting Information [Line Items]
Other current assets	758	635
Trade accounts payable and accrued expenses	(344)	(77)
Net current (liability) asset	414	558
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term asset	0	0
Total net (liability) asset	$ 414	$ 558