Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Stock-Based Compensation
Stock-based compensation expense	$ 2,300		$ 2,100		$ 4,000	$ 3,200
Proceeds from exercise of stock options					$ 2,396	$ 822
Common Stock
Stock-Based Compensation
Exercise of stock options (in shares)	104,000	288,000	46,000	75,000
Common Stock | Employee stock purchase plan
Stock-Based Compensation
Exercise of stock options (in shares)	300,000		200,000		600,000	300,000
Proceeds from exercise of stock options	$ 1,000		$ 900		$ 2,800	$ 1,300
2000 Stock Plan
Stock-Based Compensation
Number of shares of common stock available for future grant	0				0