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
	Jan. 23, 2017 USD ($) $ / shares	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 EUR (€)	Sep. 30, 2017 USD ($)	Jun. 30, 2018 EUR (€)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
License agreements
Long term investment					$ 121,381				$ 121,381		$ 134,356	[1]
Research and development expense					292,527		$ 269,557		893,719	$ 879,263
Unrealized gain (loss) on long term investments					(9,949)		23,045		(21,911)	(2,343)
Merus
License agreements
Number of independent programs | item			11
Number of preclinical discovery programs | item			2
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%
Upfront payment under license agreement		$ 120,000
Research and development expense					2,100		1,700		7,300	4,300
Merus | Accrued and other liabilities
License agreements
Accrued and other liabilities					2,100				2,100		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 | Non-U.S.
License agreements
Royalty payments on future global net sales (as a percent)			6.00%
Merus | Maximum
License agreements
Number of additional preclinical discovery programs | item			9
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 | 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					$ 63,200				$ 63,200		$ 62,100
Ownership percentage (as a percent)					14.00%				14.00%
Unrealized gain (loss) on long term investments					$ (9,600)		$ 12,900		$ 1,100	$ (9,200)
Total revenues | €						€ 6.5		€ 16.5
Net income (loss) | €						€ (4.6)		€ 13.0
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%

[1] The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements at that date.