Cash, cash equivalents, restricted cash, and available for sale marketable securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Amortized Cost
Cash and cash equivalents	$ 140,371	$ 31,530	$ 43,604
Total marketable securities, amortized cost	79,082	138,780
Total money market funds and marketable securities	218,351	169,249
Fair Value
Cash and cash equivalents	140,371	31,530	$ 43,604
Total marketable securities	79,147	138,622
Total money market funds and marketable securities	218,416	169,091
Unrealized Gains and Losses
Total marketable securities, unrealized gains	100
Total money market funds and marketable securities	100
Total marketable securities, unrealized losses	35	158
Total money market funds and marketable securities	35	158
Money market funds
Amortized Cost
Cash and cash equivalents	139,269	30,469
Fair Value
Cash and cash equivalents	139,269	30,469
U.S. Treasury notes
Amortized Cost
Marketable securities, amortized cost	77,862	137,560
Fair Value
Marketable debt securities, fair value	77,827	137,522
Unrealized Gains and Losses
Marketable securities, unrealized losses	35	38
Equity securities
Amortized Cost
Marketable equity securities, amortized cost	1,220	1,220
Fair Value
Marketable equity securities, fair value	1,320	1,100
Unrealized Gains and Losses
Equity securitiess, unrealized gains	$ 100
Equity securitiess, unrealized losses		$ 120