Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period (shares)	366,731	365,618	318,100	316,321
Granted (shares)	0	0	203,836	206,824
Exercised (shares)	(1,870)	(7,595)	(10,705)	(19,030)
Forfeited (shares)	(14,326)	(13,451)	(24,906)	(27,233)
Expired (shares)	0	0	(135,790)	(132,310)
Outstanding and exercisable, end of period (shares)	350,535	344,572	350,535	344,572