License agreements - Calithera (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Jan. 30, 2017	Mar. 31, 2017	Jan. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017		Jan. 01, 2017
License agreements
Long term investment					$ 121,381		$ 121,381		$ 134,356	[1]
Research and development expense					292,527	$ 269,557	893,719	$ 879,263
Net income					29,176	36,054	40,430	(163,513)
Unrealized gain (loss) on long term investments					(9,949)	23,045	(21,911)	(2,343)
Calithera
License agreements
Funding of future development costs (as a percent)				70.00%
Upfront payment made under license agreement				$ 45,000
Potential milestone payments to be made with profit sharing in effect				430,000
Potential milestone payments to be made with profit sharing terminated				$ 750,000
Milestone payment made under license agreement			$ 12,000
Research and development expense					2,400	2,300	8,200	5,900
Calithera | Accrued and other liabilities
License agreements
Accrued and other liabilities					1,700		1,700
Calithera | U.S.
License agreements
Percentage of profit (losses)				60.00%
Calithera
License agreements
Milestone payment made under license agreement			$ 12,000
Total consideration paid		$ 53,000
Long term investment					$ 9,000		$ 9,000		14,400
Ownership percentage (as a percent)					5.00%		5.00%
Calithera | Other Comprehensive Income
License agreements
Unrealized gain (loss) on long term investments					$ 400	1,500	$ (5,300)	15,500
Calithera | Calithera | Accrued and other liabilities
License agreements
Accrued and other liabilities									$ 900
Calithera | Stock purchase agreement
License agreements
Purchase of common stock under Stock Purchase Agreement (in shares )				1.7
Purchase price of common stock											$ 8,000
Per share price		$ 6.75									$ 4.65
Fair market value of our long term investments		$ 11,600
Stock purchase price				$ 8,000
Long term investment						$ 11,600		11,600
Research and development expense								$ 41,400
Calithera | Collaboration and license agreement
License agreements
Upfront license fees				$ 45,000
Calithera | ASU 2016-01
License agreements
Net income	$ 2,800

[1] The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements at that date.