Marketable securities (Details)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) security
Marketable securities
Aggregate estimated fair value	$ 111,977,000	$ 135,457,000
Unrealized loss position	0	$ 30,200,000
Unrealized loss position (Number of securities) | security		3
U.S. Treasury obligations
Marketable securities
Amortized cost	111,336,000	$ 135,389,000
Gross unrealized holding gains	641,000	70,000
Gross unrealized holding losses		(2,000)
Aggregate estimated fair value	$ 111,977,000	$ 135,457,000