Preferred Stock, Common Stock and Stockholders' Equity - Summary of Estimated Fair Value of Employee Stock Purchase Plan Award (Detail) - Employee stock purchase plan [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Stock-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk free interest rate, minimum	0.79%	0.47%
Risk free interest rate, maximum	1.10%	0.50%
Expected term	6 months	6 months
Expected volatility of common stock, minimum	37.60%	83.83%
Expected volatility of common stock, maximum	99.23%	212.80%
Expected dividend yield	0.00%	0.00%