Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) (Employee Stock Purchase Plan)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Employee Stock Purchase Plan
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period	433,801	497,736	433,452	425,196
Granted	0	0	246,717	327,936
Exercised	(7,478)	(24,381)	(60,483)	(45,749)
Forfeited	(14,007)	(17,781)	(36,819)	(69,494)
Expired	0	0	(170,551)	(182,315)
Outstanding and exercisable, end of period	412,316	455,574	412,316	455,574