Stockholders' Equity - Stock Options Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Number
Options outstanding, Balance (in shares)	709,668
Granted (in shares)	0
Exercised (in shares)	(205,933)
Forfeited (in shares)	0
Options outstanding, Balance (in shares)	503,735
Options exercisable at the end of the year (in shares)	498,525
Weighted average exercise price
Options outstanding, weighted average exercise price (in dollars per share)	$ 17.941
Granted, weighted average exercise price (in dollars per share)	0
Exercised, weighted average exercise price (in dollars per share)	10.352
Forfeited, weighted average exercise price (in dollars per share)	0
Options outstanding, weighted average exercise price (in dollars per share)	21.043
Options exercisable at the end of the year, weighted average exercise price (in dollars per share)	$ 21.086
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, aggregate intrinsic value	$ 19,513		$ 24,810
Options outstanding, weighted average remaining contractual life (Year)	4 years 5 months 17 days	4 years 6 months 5 days
Options exercisable at the end of the year, aggregate intrinsic value	$ 19,290
Options exercisable at the end of the year, weighted average remaining contractual life (Year)	4 years 5 months 10 days