License agreements - Calithera (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended		3 Months Ended
	Jan. 01, 2018	Jan. 30, 2017	Mar. 31, 2017	Jan. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017		Jan. 01, 2017
License agreements
Long term investment					$ 165,972		$ 134,356	[1]
Unrealized gain (loss) on long term investments					22,679	$ (5,814)
Research and development expense					303,103	407,920
Net income					(41,140)	(187,083)
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					2,600	1,600
Calithera | Accrued and other liabilities
License agreements
Accrued and other liabilities					1,500
Calithera | U.S.
License agreements
Percentage of profit (losses)				60.00%
Calithera
License agreements
Total consideration paid		$ 53,000
Long term investment					$ 10,800		14,400
Ownership percentage (as a percent)					5.00%
Calithera | Other Comprehensive Income
License agreements
Unrealized gain (loss) on long term investments					$ (3,500)	$ 8,300
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
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.