License agreements - Calithera (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 30, 2017	Jan. 01, 2017	Mar. 31, 2017	Jan. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	[1]
License agreements
Long term investment					$ 169,020		$ 169,020		$ 31,987
Unrealized Gain (Loss) on Securities					23,045	$ 24,301	(2,343)	$ 20,497
Research and development expense					269,612	$ 143,184	879,423	$ 420,276
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					2,300		5,900
Calithera | Accrued and other liabilities
License agreements
Accrued and other liabilities					2,000		2,000
Calithera | U.S.
License agreements
Percentage of profit (losses)				60.00%
Calithera
License agreements
Total consideration paid	$ 53,000
Long term investment					$ 27,100		$ 27,100
Ownership percentage (as a percent)					5.00%		5.00%
Calithera | Other Comprehensive Income
License agreements
Unrealized Gain (Loss) on Securities					$ 1,500		$ 15,500
Calithera | Stock purchase agreement
License agreements
Purchase of common stock under Stock Purchase Agreement (in shares )		1,720,430
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

[1] The condensed consolidated balance sheet at December 31, 2016 has been derived from the audited financial statements at that date.