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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, 2018 EUR (€)	Sep. 30, 2017 EUR (€)	Sep. 30, 2018 EUR (€)	Sep. 30, 2017 EUR (€)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)
License agreements
Long term investment									$ 99,199				$ 134,356
Research and development expense									1,197,957	$ 1,326,134	$ 581,861
Unrealized gain (loss) on long term investments									(44,093)	(24,275)	(3,261)
Current assets									1,832,254				1,504,854
Current liabilities									425,277				375,396
Merus
License agreements
Number of independent programs \| item			11
Upfront payment under license agreement		$ 120,000
Research and development expense									10,300	6,500
Merus \| Accrued and other liabilities
License agreements
Accrued and other liabilities									2,900				2,100
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 additional royalties			4.00%
Percentage of reverse royalty			4.00%
Merus \| Maximum \| U.S.
License agreements
Royalty payments on future global net sales (as a percent)			10.00%
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%
Total revenues \| €					€ 6.5	€ 5.7	€ 23.0	€ 15.8
Net income (loss) \| €					(10.7)	€ (13.4)	(23.7)	€ (52.7)
Current assets \| €					202.6		202.6					€ 188.1
Noncurrent assets \| €					22.1		22.1					8.7
Current liabilities \| €					38.2		38.2					27.7
Noncurrent liabilities \| €					€ 101.5		€ 101.5					€ 112.6
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							$ 80,000
Per share price \| $ / shares	$ 24.50			$ 25.00							$ 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%
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									$ 44,800				$ 62,100
Ownership percentage (as a percent)									14.00%
Unrealized gain (loss) on long term investments									$ (17,300)	$ (10,700)