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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		12 Months Ended
	Jan. 23, 2017 USD ($) $ / shares	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 EUR (€)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)
License agreements
Long term investment					$ 115,563			$ 115,563			$ 99,199
Research and development expense					289,363		$ 298,089	559,908	$ 601,192
Unrealized gain (loss) on long term investments					(4,625)		(34,641)	16,364	(11,962)
Current assets					2,058,625			2,058,625			1,832,254
Current liabilities					367,105			367,105			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					1,700		2,800	4,300	5,200
Merus \| Accrued and other liabilities
License agreements
Accrued and other liabilities					1,600			1,600			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					$ 46,900			$ 46,900			$ 44,800
Ownership percentage (as a percent)					14.00%			14.00%
Unrealized gain (loss) on long term investments					$ (400)		$ 13,500	$ 2,100	$ 10,800
Total revenues \| €						€ 7.7
Net loss \| €						€ (6.2)
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%