Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period (shares)	390,776	366,731	328,533	318,100
Granted (shares)	0	0	235,572	203,836
Exercised (shares)	(4,386)	(1,870)	(19,016)	(10,705)
Forfeited (shares)	(23,172)	(14,326)	(39,797)	(24,906)
Expired (shares)	0	0	(142,074)	(135,790)
Outstanding and exercisable, end of period (shares)	363,218	350,535	363,218	350,535