Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2020 USD ($) bank	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2020 USD ($) segment bank	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Apr. 01, 2019 USD ($)
Accounting Policies [Abstract]
Net Loss	$ (64,913)	$ (85,604)	$ (70,568)	$ (67,904)	$ (75,014)	$ (70,633)	$ (65,770)	$ (62,134)	$ (288,989)	$ (273,551)	$ (143,255)
Net cash used in operating activities									$ (221,172)	$ (224,088)	$ (117,255)
Number of operating segments | segment									1
Number of reporting segments | segment									1
Number of banking institutions | bank	3								3
Expected dividend yield									0.00%	0.00%	0.00%
Operating lease right-of-use asset	$ 11,146								$ 11,146			$ 9,400
Operating lease liability	1,516								1,516			800
Long-term operating lease liability	$ 10,996								$ 10,996			9,800
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred rent liability												$ (1,200)
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful lives of property and equipment									3 years
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful lives of property and equipment									7 years