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License agreements - Merus (Details) $ / shares in Units, $ in Thousands, € in Millions, shares in Millions		1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 23, 2017 USD ($) $ / shares	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2019 EUR (€)	Sep. 30, 2018 EUR (€)	Sep. 30, 2019 EUR (€)	Sep. 30, 2018 EUR (€)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2018 EUR (€)
License agreements
Long term investments									$ 133,657	$ 99,199
Research and development expense									1,154,111	1,197,957	$ 1,326,134
Unrealized gain (loss) on long term investments									34,458	(44,093)	(24,275)
Current assets									2,481,488	1,832,254
Current liabilities									513,340	425,277
Merus
License agreements
Number of independent programs \| item			11
Upfront payment under license agreement		$ 120,000
Research and development expense									7,200	10,300	6,500
Merus \| Accrued and other liabilities
License agreements
Accrued and other liabilities									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
Number of programs under the resulting products are co-funded for development \| item			2
Associated future global development costs , if elected to co-develop (as a percent)			35.00%
Fair market value of our long term investments									$ 45,100	44,800
Ownership percentage (as a percent)									11.00%
Unrealized gain (loss) on long term investments									$ 300	$ (17,300)	10,700
Total revenues \| €					€ 8.1	€ 6.5	€ 21.4	€ 23.0
Net loss \| €					(8.3)	€ (10.7)	(26.4)	€ (23.7)
Current assets \| €					166.7		166.7					€ 195.6
Noncurrent assets \| €					23.1		23.1					22.9
Current liabilities \| €					29.7		29.7					29.0
Noncurrent liabilities \| €					€ 89.8		€ 89.8					€ 97.7
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	$ 24.50			$ 25.00
Discount for lack of marketability	$ 5,600
Fair value of shares on the issuance date	72,800
Total consideration paid	$ 80,000
Long term investments											72,800
Research and development expense											$ 7,200