Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Apr. 01, 2019 USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment							1
Number of reporting segments | segment							1
Net loss	$ 85,604	$ 70,568	$ 67,904	$ 70,633	$ 65,770	$ 62,134	$ 224,076	$ 198,537
Cash and cash equivalents used in operations							201,943	$ 170,980
Operating lease right-of-use asset	11,491						11,491		$ 9,400
Operating lease liability	1,449						1,449		800
Long-term operating lease liability	$ 11,399						$ 11,399		9,800
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred rent obligations									$ (1,200)