Nature of the Business and Basis of Presentation - Nature of Business (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nature of the Business and Basis of Presentation
Cash and cash equivalents	$ 54,437	$ 54,062	$ 41,538
Accumulated deficit	$ 383,615	$ 297,243