License agreements - Merus (Details) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended			3 Months Ended
	Jan. 27, 2017 USD ($) $ / shares	Jan. 23, 2017 USD ($)	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) $ / shares shares	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
License agreements
Long term investment						$ 165,972		$ 134,356	[1]
Research and development expense						303,103	$ 407,920
Unrealized gain (loss) on long term investments						22,679	(5,814)
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
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				2,400	1,100
Fair market value of our long term investments						$ 59,300		62,100
Ownership percentage (as a percent)						16.00%
Unrealized gain (loss) on long term investments						$ (2,800)	$ 1,900
Merus | Accrued and other liabilities
License agreements
Accrued and other liabilities						$ 4,500		$ 2,100
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
Standstill period						3 years
Lock-up period		18 months
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.