License agreements - Calithera (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions		1 Months Ended		3 Months Ended		12 Months Ended
	Jan. 30, 2017	Mar. 31, 2017	Jan. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2019		Jan. 01, 2017
License agreements
Long term investments				$ 180,993			$ 133,657	[1]
Research and development expense				1,085,287	$ 270,545
Net income				(720,642)	102,312
Unrealized gain (loss) on long term investments				(48,132)	20,989
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				2,500	4,600
Calithera | Accrued and other liabilities
License agreements
Accrued and other liabilities				1,300
Calithera | U.S.
License agreements
Percentage of profit (losses)			60.00%
Calithera | Other Comprehensive Income
License agreements
Unrealized gain (loss) on long term investments				(2,200)	$ (4,700)
Calithera | Calithera | Accrued and other liabilities
License agreements
Accrued and other liabilities							1,100
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			$ 7,600			$ 9,800
Total consideration paid	53,000
Upfront license fees	45,000
Stock purchase price	$ 8,000
Long term investments						$ 11,600
Research and development expense						$ 41,400
Ownership percentage (as a percent)				3.00%

[1] The condensed consolidated balance sheet at December 31, 2019 has been derived from the audited consolidated financial statements at that date.