Stockholders' Equity - Preferred Stock and Convertible Preferred Stock (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Preferred stock
Class of Stock [Line Items]
Preferred stock	$ 0	$ 0
Series B Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock	1	1
Series D Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock	14	17
Series E Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock	7	0
Series F Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock	$ 8	$ 0