Convertible Notes, Convertible Preferred Stock and Common and Preferred Stock Warrants - Common Stock Warrants (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended	9 Months Ended		12 Months Ended
	Aug. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2013	Jul. 01, 2017	Sep. 30, 2016	Dec. 31, 2014
Class of Warrant or Right [Line Items]
Issuance of common stock upon exercise of warrants (in shares)			312,575
Adjustment to additional paid in capital related to warrant reclassification		$ 9,410		$ 9,410
Common stock warrants
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)										33,333
Warrant exercise price (in dollars per share)										$ 38.64
Revaluation of warrants						$ 9,200	$ 3,300
Common stock warrants | Maximum | J.P. Morgan
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)						146,666
Common stock warrants | Maximum | CPPIB
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)						166,222
Convertible promissory notes | Convertible Promissory Notes due December 2020, Recourse
Class of Warrant or Right [Line Items]
Interest rate percentage			6.00%		6.00%			6.00%
Affiliated entity
Class of Warrant or Right [Line Items]
Adjustment to additional paid in capital related to warrant reclassification	$ 9,400
Affiliated entity | Common stock warrants
Class of Warrant or Right [Line Items]
Revaluation of warrants					$ 200
Affiliated entity | Common stock warrants | CPPIB
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)	312,888
Affiliated entity | Common stock warrants | Maximum | J.P. Morgan
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)						146,666
Affiliated entity | Common stock warrants | Maximum | CPPIB
Class of Warrant or Right [Line Items]
Number of securities called by warrants (in shares)						166,222
Affiliated entity | Convertible promissory notes | Convertible Promissory Notes due December 2020, Recourse
Class of Warrant or Right [Line Items]
Interest rate percentage						6.00%		6.00%	5.00%