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License agreements - Merus (Details) $ / shares in Units, $ in Thousands, € in Millions, shares in Millions		1 Months Ended			3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Jan. 23, 2017 USD ($) $ / shares	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2019 USD ($)	Jun. 30, 2019 EUR (€)	Sep. 30, 2018 USD ($)	Jun. 30, 2019 EUR (€)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)
License agreements
Long term investment					$ 117,902				$ 117,902			$ 99,199
Research and development expense					281,336		$ 292,527		841,244	$ 893,719
Unrealized gain (loss) on long term investments					2,339		(9,949)		18,703	(21,911)
Current assets					2,319,595				2,319,595			1,832,254
Current liabilities					461,683				461,683			425,277
Merus
License agreements
Associated future global development costs , if elected to co-develop (as a percent)			35.00%
Upfront payment under license agreement		$ 120,000
Research and development expense					1,400		2,100		5,700	7,300
Merus \| Accrued and other liabilities
License agreements
Accrued and other liabilities					1,500				1,500			2,900
Merus \| U.S.
License agreements
Profit sharing (as a percent)			50.00%
Percentage of profit (losses)			50.00%
Merus \| Minimum
License agreements
Royalty payments on future global net sales (as a percent)			6.00%
Merus \| Minimum \| U.S.
License agreements
Royalty payments on future global net sales (as a percent)			6.00%
Merus \| Minimum \| Non-U.S.
License agreements
Royalty payments on future global net sales (as a percent)			6.00%
Merus \| Maximum
License agreements
Royalty payments on future global net sales (as a percent)			10.00%
Percentage of reverse royalty			4.00%
Merus \| Maximum \| U.S.
License agreements
Royalty payments on future global net sales (as a percent)			10.00%
Percentage of additional royalties			4.00%
Merus \| Maximum \| Non-U.S.
License agreements
Royalty payments on future global net sales (as a percent)			10.00%
Merus \| Maximum \| Development and Regulatory Milestones
License agreements
Additional milestone payments under the license agreement			$ 100,000
Merus \| Maximum \| Commercialization Milestones
License agreements
Additional milestone payments under the license agreement			$ 250,000
Merus
License agreements
Per share price of common stock \| $ / shares	$ 24.50
Discount for lack of marketability	$ 5,600
Fair value of shares on the issuance date	72,800
Total consideration paid	80,000
Long term investment	$ 72,800
Research and development expense											$ 7,200
Fair market value of our long term investments					$ 57,000				$ 57,000			$ 44,800
Ownership percentage (as a percent)					14.00%				14.00%
Unrealized gain (loss) on long term investments					$ 10,100		$ (9,600)		$ 12,200	$ 1,100
Total revenues \| €						€ 5.6		€ 13.3
Net loss \| €						€ (12.0)		€ (18.2)
Merus \| Stock purchase agreement
License agreements
Purchase of common stock under Stock Purchase Agreement (in shares ) \| shares				3.2
Purchase price of common stock				$ 80,000
Per share price \| $ / shares				$ 25.00
Lock-up period	18 months
Standstill period									3 years
Percentage of total shares allowed to sell during any 12-month period (as a percent)	33.00%
Percentage of total shares allowed to sell during any three-month period (as a percent)	10.00%