Balance Sheet Amounts Associated With Medicare Part D (Detail) - USD ($) $ in Millions	Oct. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Other current assets		$ 5,767	$ 4,007
Trade accounts payable and accrued expenses		(2,216)	(2,095)
Other long-term assets		2,035	1,583
Other long-term liabilities		(225)	(236)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets		107	105
Trade accounts payable and accrued expenses		(11)	(36)
Net current asset		96	69
Other long-term assets		53	0
Other long-term liabilities		(11)	0
Net long-term asset		42	0
Total net (liability) asset		138	69
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets		2,763	1,690
Trade accounts payable and accrued expenses		(66)	(32)
Net current asset		2,697	1,658
Other long-term assets		0	0
Other long-term liabilities		0	0
Net long-term asset		0	0
Total net (liability) asset		$ 2,697	$ 1,658
2014 Coverage Year | Subsequent Event | CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Proceeds from settlements	$ 1,700
Reinsurance and low-income cost subsidies | 2014 Coverage Year | Subsequent Event | CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Proceeds from settlements	$ 1,600