Share-Based Payments - Fair Value of Stock Option Granted using Black-Scholes Option-Pricing Model Assumptions - Non-employees (Details) - Non-Employee Stock Options	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum (as a percent)	1.50%	1.08%	1.19%
Risk-free interest rate, maximum (as a percent)	2.32%	2.38%	2.26%
Expected dividend yield (as a percent)	0.00%	0.00%	0.00%
Expected option life	10 years	10 years	10 years
Expected volatility, minimum (as a percent)	74.40%	69.92%	67.24%
Expected volatility, maximum (as a percent)	84.31%	92.09%	92.40%