License agreements - Merus (Details) $ / shares in Units, $ in Thousands, € in Millions, shares in Millions		1 Months Ended			3 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	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($)
License agreements
Long term investments					$ 180,993				$ 133,657	[1]
Research and development expense					1,085,287	$ 270,545
Unrealized gain (loss) on long term investments					(48,132)	20,989
Current assets					1,716,169				2,481,488	[1]
Current liabilities					503,412				513,340	[1]
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			1
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					2,300	2,600
Merus | Accrued and other liabilities
License agreements
Accrued and other liabilities					2,200				1,600
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
Number of additional preclinical discovery programs | item			10
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
Total revenues | €							€ 31.1
Net loss | €							€ (55.2)
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
Fair market value of our long term investments					$ 38,700				$ 45,100
Ownership percentage (as a percent)					11.00%
Unrealized gain (loss) on long term investments					$ (6,300)	$ (2,500)

[1] The condensed consolidated balance sheet at December 31, 2019 has been derived from the audited consolidated financial statements at that date.