Balance Sheet Amounts Associated With Medicare Part D (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Other current assets	$ 5,568	$ 4,712
Trade accounts payable and accrued expenses	(4,074)	(2,212)
Other long-term assets	2,465	2,475
Other long-term liabilities	(278)	(235)
Risk Corridor Settlement
Segment Reporting Information [Line Items]
Other current assets	33	25
Trade accounts payable and accrued expenses	(44)	(47)
Net current (liability) asset	(11)	(22)
Other long-term assets	49	0
Other long-term liabilities	(29)	0
Net long-term asset	20	0
Total net (liability) asset	9	(22)
CMS Subsidies/ Discounts
Segment Reporting Information [Line Items]
Other current assets	2,408	2,082
Trade accounts payable and accrued expenses	(704)	(63)
Net current (liability) asset	1,704	2,019
Other long-term assets	0	0
Other long-term liabilities	0	0
Net long-term asset	0	0
Total net (liability) asset	$ 1,704	$ 2,019