Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period (shares)	316,321	340,958
Granted (shares)	206,824	198,450
Exercised (shares)	(11,435)	(26,689)
Forfeited (shares)	(13,782)	(7,682)
Expired (shares)	(132,310)	(143,082)
Outstanding and exercisable, end of period (shares)	365,618	361,955