Balance Sheet Amounts Associated With Medicare Part D (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Other current assets	$ 1,615	$ 1,027
Trade accounts payable and accrued expenses	(1,962)	(1,783)
Risk Corridor Settlements
Segment Reporting Information [Line Items]
Other current assets	38	2
Trade accounts payable and accrued expenses	(428)	(331)
Net current (liability) asset	(390)	(329)
CMS Subsidies
Segment Reporting Information [Line Items]
Other current assets	654	363
Trade accounts payable and accrued expenses	(155)	(139)
Net current (liability) asset	$ 499	$ 224