Nature of Business and Significant Accounting Policies - Additional Information (Details)		12 Months Ended
	Mar. 13, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) Product Segment Unit	Dec. 31, 2017 USD ($)
Nature Of Business [Line Items]
Number of revenue-generating product | Product		1
Warrants outstanding, term	10 years
Long-term restricted cash		$ 7,448,000	$ 10,000,000
Number of operating segment | Segment		1
Number of reporting unit | Unit		1
ASU 2016-18
Nature Of Business [Line Items]
Restricted cash		$ 7,800,000	$ 10,400,000
Stock Option
Nature Of Business [Line Items]
Expected dividend yield		0.00%	0.00%
Estimated forfeiture rate		10.00%
RSUs and Options
Nature Of Business [Line Items]
Estimated forfeiture rate		10.00%
Letter of Credit
Nature Of Business [Line Items]
Long-term restricted cash		$ 2,400,000
Covenant of MidCap Loan Agreement
Nature Of Business [Line Items]
Long-term restricted cash		$ 5,000,000
Subsequent Event
Nature Of Business [Line Items]
Proceeds from issuance of warrants	$ 22,000,000
Number of common stock to be issued up conversion of warrants | shares	19,850,000
Number of warrants issued | shares	19,850,000
Warrants exercise price, per share | $ / shares	$ 1.30
Number of common stock to be issued exercise of prefunded warrants | shares	2,150,000
Pre funded warrants exercise price, per share | $ / shares	$ 0.01
Additional proceeds to be received upon exercise of the warrants	$ 28,600,000
Maximum
Nature Of Business [Line Items]
Cash equivalents, maturity period		90 days
Maximum eligible annual benefit under the program		$ 12,000
Maximum amount of benefit provided to customers		$ 100,000
Maximum | Subsequent Event
Nature Of Business [Line Items]
Number of common stock to be issued up conversion of warrants | shares	2,150,000