Medicare Part D - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	$ 2,010	$ 2,042
Other current assets	2,232	1,670
Risk Corridor Settlements
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	261	393
Other current assets	58	37
Risk Corridor Settlements | 2012 Contract | Expected to be settled in the fourth quarter of 2013
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	210
Risk Corridor Settlements | 2011 Contract
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses		158
Payment date		2013-01
CMS Subsidies/Discounts
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	131	77
Other current assets	880	635
CMS Subsidies/Discounts | 2012 Contract | Expected to be settled in the fourth quarter of 2013
Segment Reporting Information [Line Items]
Other current assets	$ 539