Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period (shares)	318,100	316,321
Granted (shares)	203,836	206,824
Exercised (shares)	(8,835)	(11,435)
Forfeited (shares)	(10,580)	(13,782)
Expired (shares)	(135,790)	(132,310)
Outstanding and exercisable, end of period (shares)	366,731	365,618