Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Stock-Based Compensation
Stock-based compensation expense	$ 2,100	$ 1,500	$ 3,200	$ 2,600
Proceeds from exercise of stock options			$ 822	$ 9,290
Common Stock
Stock-Based Compensation
Exercise of stock options (in shares)	200,000	1,200,000	300,000	1,700,000
Proceeds from exercise of stock options	$ 900	$ 7,100	$ 1,300	$ 9,600
2000 Stock Plan
Stock-Based Compensation
Number of shares of common stock available for future grant	0		0