Medicare Part D - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	$ 2,042	$ 1,783
Risk Corridor Settlements
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	393	331
Risk Corridor Settlements | 2011 Contract
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	$ 158
Payment date	2013-01