Description of Business - Additional Information (Details) - USD ($)		9 Months Ended
	Jul. 27, 2018	Sep. 30, 2018	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Total proceeds from issuance of common stock from follow on underwritten public offering		$ 232,846,000	$ 108,910,000
Common Stock A | Follow On Underwritten Public Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock	3,450,000
Gross proceeds from issuance of common stock from option granted to the underwriters	$ 248,400,000
Total proceeds from issuance of common stock from follow on underwritten public offering	$ 232,846,000