Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period (shares)	350,535	344,572	318,100	316,321
Granted (shares)	0	0	203,836	206,824
Exercised (shares)	(2,531)	(5,452)	(13,236)	(24,482)
Forfeited (shares)	(9,583)	(9,291)	(34,489)	(36,524)
Expired (shares)	0	0	(135,790)	(132,310)
Outstanding and exercisable, end of period (shares)	338,421	329,829	338,421	329,829