Balance Sheet Amounts Associated With Medicare Part D (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Other current assets	$ 1,670	$ 1,027
Trade accounts payable and accrued expenses	(2,042)	(1,783)
Risk Corridor Settlements
Segment Reporting Information [Line Items]
Other current assets	37	2
Trade accounts payable and accrued expenses	(393)	(331)
Net current (liability) asset	(356)	(329)
CMS Subsidies/Discounts
Segment Reporting Information [Line Items]
Other current assets	635	363
Trade accounts payable and accrued expenses	(77)	(139)
Net current (liability) asset	$ 558	$ 224