License agreements - Mercus (Details) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended				3 Months Ended
	Jan. 27, 2017 USD ($) $ / shares	Jan. 23, 2017	Feb. 28, 2017 USD ($)	Jan. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) $ / shares shares		Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
License agreements
Long term investment					$ 31,987	[1]	$ 158,322
Research and development expense							407,972	$ 156,824
Unrealized Gain (Loss) on Securities							(5,814)	$ (2,950)
Merus
License agreements
Number of independent programs | item				11
Number of preclinical discovery programs | item				2
Number of programs under the resulting products are cofunded 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						1,100
Fair market value of our long term investments							$ 74,700
Ownership percentage (as a percent)							17.00%
Unrealized Gain (Loss) on Securities							$ 1,900
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, 2016 has been derived from the audited financial statements at that date.