Description of Business and Summary of Significant Accounting Policies - Narrative (Details) $ / shares in Units, $ in Thousands			9 Months Ended		12 Months Ended
	Jul. 23, 2018 USD ($) $ / shares shares	Aug. 02, 2017 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock issued upon conversion (in shares) | shares		55,422,002
Conversion basis		1
Initial public offering costs					$ 3,708
Proceeds from issuance of convertible senior notes			$ 138,953	$ 0
Proceeds from issuance of convertible debt, net of underwriting discounts	$ 138,953
Issuance costs of common stock and debt offerings			862
Accumulated deficit			158,759		129,003
Additional Paid-in Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Initial public offering costs					$ 3,708
IPO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shares sold in offering (in shares) | shares		10,615,650
Offering price (in dollars per share) | $ / shares		$ 15.00
Net proceeds from stock offering		$ 144,380
IPO | Additional Paid-in Capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Initial public offering costs			$ 3,708
Public Offerings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shares sold in offering (in shares) | shares	4,836,336
Offering price (in dollars per share) | $ / shares	$ 23.50
Net proceeds from stock offering	$ 107,593
Over-Allotment Option
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shares sold in offering (in shares) | shares	630,826	1,384,650
1.75% Convertible Senior Notes due 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from issuance of convertible senior notes	$ 143,750
Stated interest rate	1.75%
Proceeds from issuance of convertible debt, net of underwriting discounts	$ 138,953
1.75% Convertible Senior Notes due 2023 | Over-Allotment Option
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Proceeds from issuance of convertible senior notes	$ 18,750