Segment Information - Product Sales (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Total product sales	$ 25,662	$ 29,953	$ 32,151
Harvoni [Member]
Revenue from External Customer [Line Items]
Total product sales	4,370	9,081	13,864
Genvoya [Member]
Revenue from External Customer [Line Items]
Total product sales	3,674	1,484	45
Epclusa [Member]
Revenue from External Customer [Line Items]
Total product sales	3,510	1,752	0
Truvada [Member]
Revenue from External Customer [Line Items]
Total product sales	3,134	3,566	3,459
Atripla [Member]
Revenue from External Customer [Line Items]
Total product sales	1,806	2,605	3,134
Descovy [Member]
Revenue from External Customer [Line Items]
Total product sales	1,218	298	0
Stribild [Member]
Revenue from External Customer [Line Items]
Total product sales	1,053	1,914	1,825
Viread [Member]
Revenue from External Customer [Line Items]
Total product sales	1,046	1,186	1,108
Odefsey [Member]
Revenue from External Customer [Line Items]
Total product sales	1,106	329	0
Complera Eviplera [Member]
Revenue from External Customer [Line Items]
Total product sales	966	1,457	1,427
Sovaldi [Member]
Revenue from External Customer [Line Items]
Total product sales	964	4,001	5,276
Vosevi [Member]
Revenue from External Customer [Line Items]
Total product sales	293	0	0
Other antiviral products [Member]
Revenue from External Customer [Line Items]
Total product sales	196	72	69
Antiviral Products [Member]
Revenue from External Customer [Line Items]
Total product sales	23,336	27,745	30,207
Letairis [Member]
Revenue from External Customer [Line Items]
Total product sales	887	819	700
Ranexa [Member]
Revenue from External Customer [Line Items]
Total product sales	717	677	588
Ambisome [Member]
Revenue from External Customer [Line Items]
Total product sales	366	356	350
Zydelig [Member]
Revenue from External Customer [Line Items]
Total product sales	149	168	132
Other Products [Member]
Revenue from External Customer [Line Items]
Total product sales	$ 207	$ 188	$ 174