Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Stock-Based Compensation
Stock-based compensation expense	$ 2,400	$ 1,700	$ 5,600	$ 4,300
Proceeds from exercise of stock options			$ 1,289	$ 11,112
Common Stock
Stock-Based Compensation
Exercise of stock options (in shares)	100,000	300,000	300,000	1,900,000
Proceeds from exercise of stock options	$ 500	$ 1,800	$ 1,700	$ 11,400
2000 Stock Plan
Stock-Based Compensation
Number of shares of common stock available for future grant	0		0