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License agreements - Calithera (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions		1 Months Ended		3 Months Ended						9 Months Ended		12 Months Ended
	Jan. 30, 2017	Mar. 31, 2017	Jan. 31, 2017	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017	Dec. 31, 2019	Jan. 01, 2017
License agreements
Long term investments				$ 222,810						$ 222,810			$ 133,657
Research and development expense				438,109			$ 281,336			1,809,997	$ 841,244
Net income				(15,203)	$ 290,298	$ (720,642)	128,271	$ 105,318	$ 102,312	(445,547)	335,901
Unrealized gain (loss) on long term investments				(13,207)			2,339			10,935	18,703
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	738,000						738,000
Milestone payment made under license agreement		$ 12,000
Research and development expense				2,000			4,700			6,400	14,700
Calithera \| Accrued and other liabilities
License agreements
Accrued and other liabilities				500						500
Calithera \| U.S.
License agreements
Percentage of profit (losses)			60.00%
Calithera
License agreements
Total consideration paid	$ 53,000		$ 53,000
Long term investments				$ 5,900						$ 5,900			9,800
Ownership percentage (as a percent)				2.00%						2.00%
Calithera \| Other Comprehensive Income
License agreements
Unrealized gain (loss) on long term investments				$ (3,200)			$ (1,400)			$ (3,900)	$ (1,600)
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
Stock purchase price			$ 8,000
Long term investments												$ 11,600
Research and development expense												$ 41,400
Calithera \| Collaboration and license agreement
License agreements
Upfront license fees			$ 45,000