Medicare Part D - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	$ 2,174	$ 2,042
Risk Corridor Settlements
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses	217	393
Risk Corridor Settlements | 2011 Contract
Segment Reporting Information [Line Items]
Trade accounts payable and accrued expenses		$ 158
Payment date		2013-01