Summary of significant accounting policies - Marketable Securities - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Cash and cash equivalents	$ 46,859,000	$ 31,530,000	$ 36,641,000
Total marketable securities, amortized cost	110,171,000	138,780,000
Total money market funds and marketable securities	156,344,000	169,249,000
Fair Value
Total marketable securities	109,575,000	138,622,000
Total money market funds and marketable securities	155,748,000	169,091,000
Unrealized Gains and Losses
Total marketable securities, unrealized gains	1,000
Total money market funds and marketable securities	1,000
Total marketable securities, unrealized losses	597,000	158,000
Total money market funds and marketable securities	597,000	$ 158,000
Other than temporary losses	$ 0
Maximum
Unrealized Gains and Losses
Debt securities, contractual maturity period	1 year	1 year
Money market funds
Amortized Cost
Cash and cash equivalents	$ 46,173,000	$ 30,469,000
Fair Value
Cash and cash equivalents	46,173,000	30,469,000
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	108,951,000	137,560,000
Fair Value
Marketable debt securities, fair value	108,947,000	137,522,000
Unrealized Gains and Losses
Marketable securities, unrealized gains	1,000
Marketable securities, unrealized losses	5,000	38,000
Equity securities
Amortized Cost
Marketable equity securities, amortized cost	1,220,000	1,220,000
Fair Value
Marketable equity securities, fair value	628,000	1,100,000
Unrealized Gains and Losses
Equity securitiess, unrealized losses	$ 592,000	$ 120,000