Cash, cash equivalents, and available-for-sale marketable securities (Details) - USD ($)	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost
Cash and cash equivalents	$ 82,640,000	$ 46,859,000
Marketable securities, amortized cost	225,867,000	110,171,000
Total money market funds and marketable securities	307,578,000	156,344,000
Fair Value
Marketable securities, fair value	226,784,000	109,575,000
Total money market funds and marketable securities	308,495,000	155,748,000
Unrealized Gains and Losses
Marketable securities, unrealized gains	917,000	1,000
Total money market funds and marketable securities	917,000	1,000
Marketable securities, unrealized losses		597,000
Total money market funds and marketable securities		597,000
Other than temporary losses	$ 0
Maximum
Unrealized Gains and Losses
Debt securities, contractual maturity period	1 year
Money market funds
Amortized Cost
Cash and cash equivalents	$ 81,711,000	46,173,000
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	224,647,000	108,951,000
Fair Value
Marketable securities, fair value	224,760,000	108,947,000
Unrealized Gains and Losses
Marketable securities, unrealized gains	113,000	1,000
Marketable securities, unrealized losses		5,000
Equity securities
Amortized Cost
Marketable securities, amortized cost	1,220,000	1,220,000
Fair Value
Marketable securities, fair value	2,024,000	628,000
Unrealized Gains and Losses
Marketable securities, unrealized gains	$ 804,000
Marketable securities, unrealized losses		$ 592,000