Stockholders' Equity - Stock Options Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, Balance (in shares)	709,668
Granted (in shares)	0
Exercised (in shares)	(149,132)
Forfeited (in shares)	0
Options outstanding, Balance (in shares)	560,536
Options exercisable at the end of the year (in shares)	548,421
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Options outstanding, weighted average exercise price (in dollars per share)	$ 17.941
Granted, weighted average exercise price (in dollars per share)	0.000
Exercised, weighted average exercise price (in dollars per share)	5.503
Forfeited, weighted average exercise price (in dollars per share)	0.000
Options outstanding, weighted average exercise price (in dollars per share)	21.250
Options exercisable at the end of the year, weighted average exercise price (in dollars per share)	$ 21.343
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, aggregate intrinsic value	$ 21,514		$ 24,810
Options outstanding, weighted average remaining contractual life (Year)	5 years 25 days	4 years 6 months 5 days
Options exercisable at the end of the year, aggregate intrinsic value	$ 20,997
Options exercisable at the end of the year, weighted average remaining contractual life (Year)	5 years 11 days