Stockholders' Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended			12 Months Ended
	Mar. 31, 2020	Jan. 31, 2020	Apr. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 10, 2019	Jan. 31, 2017	Jan. 25, 2016
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized							600,000,000	600,000,000		600,000,000
Preferred stock, shares authorized							2,000,000	2,000,000		2,000,000
Common stock, shares outstanding							32,352,416	19,222,410
Sales per share price range												$ 111.20
Proceeds from shares sold							$ 97,890	$ 100,060	$ 63,425
Common Stock [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold							12,980,177	2,941,438	2,544,495
Proceeds from shares sold							$ 130	$ 29	$ 25
December 2018 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold							10,500,000
Proceeds from shares sold							$ 59,500
Shelf Registration Statement, Authorized Common Stock Issuance, Value							$ 100,000
December 2018 Sales Agreement [Member] | Subsequent Event [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold				7,200,000
Proceeds from shares sold				$ 38,500
Shelf Registration Statement, Maximum Authorized Common Stock Issuance, Value	$ 500			500
Public Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Sales per share price range			$ 33.00
Shares sold			1,700,000
Common stock issued upon exercise in full over-allotment granted			200,000
Proceeds from shares sold			$ 54,000
Payments of Stock Issuance Costs			$ 3,600
December 2017 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold					2,500,000			900,000
Proceeds from shares sold					$ 37,900			$ 35,900
Shelf Registration Statement, Authorized Common Stock Issuance, Value									$ 75,000
January 2017 Sales Agreement [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold						300,000			2,500,000
Proceeds from shares sold						$ 10,300			$ 63,400
Shelf Registration Statement, Authorized Common Stock Issuance, Value											$ 75,000
January 2020 Sales Agreement [Member] | Subsequent Event [Member]
Subsidiary, Sale of Stock [Line Items]
Proceeds from shares sold		$ 100,000
January 2020 Sales Agreement [Member] | Subsequent Event [Member] | Common Stock [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold		10,500,000
Proceeds from shares sold		$ 98,700
March 2020 Sales Agreement [Member] | Subsequent Event [Member]
Subsidiary, Sale of Stock [Line Items]
Proceeds from shares sold	$ 150,000
March 2020 Sales Agreement [Member] | Subsequent Event [Member] | Common Stock [Member]
Subsidiary, Sale of Stock [Line Items]
Shares sold	1,500,000
Proceeds from shares sold	$ 18,600
Stock Issued During Period, Value, Other	$ 131,100			$ 131,100
Maximum [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares outstanding										469,453,883
Minimum [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares outstanding										23,472,574