Stockholders' Equity (Deficit) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended					6 Months Ended	9 Months Ended
	Nov. 03, 2020	Jun. 30, 2020	Jan. 31, 2020	Dec. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	May 31, 2020	Dec. 31, 2018	Dec. 31, 2017
Stockholders' Equity
Proceeds from shares sold, net					$ 57,190		$ 12,588			$ 450,953	$ 67,830
Market Issuance Sales Agreements
Stockholders' Equity
Issued (in shares)									8,600,000
Proceeds from shares sold, net									$ 148,100
Maximum potential proceeds						$ 150,000
May 2020 Issuance Sales Agreement
Stockholders' Equity
Issued (in shares)	700,000									2,800,000
Proceeds from shares sold, net	$ 74,100			$ 3,900						$ 160,300
Maximum potential proceeds	$ 8,700											$ 250,000
January 2020 Issuance Sales Agreement
Stockholders' Equity
Issued (in shares)						10,500,000
Proceeds from shares sold, net						$ 98,700
Maximum potential proceeds			$ 100,000
December 2018 Issuance Sales Agreement
Stockholders' Equity
Issued (in shares)			7,200,000			7,200,000		1,700,000			3,600,000
Proceeds from shares sold, net			$ 38,500			$ 38,500		$ 29,300
Maximum potential proceeds													$ 100,000
December 2017 Issuance Sales Agreement
Stockholders' Equity
Issued (in shares)								2,500,000
Proceeds from shares sold, net								$ 37,900
Maximum potential proceeds														$ 75,000
Member of the Board
Stockholders' Equity
Issued (in shares)		32,916
Share price (per share)		$ 45.57							$ 45.57
Gross proceeds from sale of common stock		$ 1,500