Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period (shares)	363,218	350,535	328,533	318,100
Granted (shares)	0	0	235,572	203,836
Exercised (shares)	(9,927)	(2,531)	(28,943)	(13,236)
Forfeited (shares)	(11,762)	(9,583)	(51,559)	(34,489)
Expired (shares)	0	0	(142,074)	(135,790)
Outstanding and exercisable, end of period (shares)	341,529	338,421	341,529	338,421