Balance Sheet Amounts Associated With Medicare Part D (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Other current assets	$ 2,232	$ 1,670
Trade accounts payable and accrued expenses	(2,010)	(2,042)
Risk Corridor Settlements
Segment Reporting Information [Line Items]
Other current assets	58	37
Trade accounts payable and accrued expenses	(261)	(393)
Net current (liability) asset	(203)	(356)
CMS Subsidies/Discounts
Segment Reporting Information [Line Items]
Other current assets	880	635
Trade accounts payable and accrued expenses	(131)	(77)
Net current (liability) asset	$ 749	$ 558