Stockholders' Equity - Stock Options Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 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)	(172,923)
Forfeited (in shares)	0
Options outstanding, Balance (in shares)	536,745
Options exercisable at the end of the year (in shares)	528,411
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
Exercised, weighted average exercise price (in dollars per share)	7.923
Forfeited, weighted average exercise price (in dollars per share)	0
Options outstanding, weighted average exercise price (in dollars per share)	21.168
Options exercisable at the end of the year, weighted average exercise price (in dollars per share)	$ 21.236
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, aggregate intrinsic value	$ 21,884		$ 24,810
Options outstanding, weighted average remaining contractual life (Year)	4 years 9 months 17 days	4 years 6 months 5 days
Options exercisable at the end of the year, aggregate intrinsic value	$ 21,509
Options exercisable at the end of the year, weighted average remaining contractual life (Year)	4 years 9 months 7 days