Nature of Business (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 31, 2013 item	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Nature of Business
Number of product candidates | item	2
Net loss		$ 17,686	$ 10,016	$ 48,557	$ 23,178
Accumulated deficit		159,095		159,095		$ 110,252
Carrying amount of cash and cash equivalents and short term investments		$ 272,386		272,386		$ 148,082
Net proceeds from issuance of common stock				$ 162,899