Stockholders' Equity - Stock Options Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, Balance (in shares)	1,456,285
Granted (in shares)	0
Exercised (in shares)	(611,220)
Forfeited (in shares)	(1,234)
Options outstanding, Balance (in shares)	843,831	1,456,285
Options exercisable at the end of the year (in shares)	765,242
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)	$ 16.172
Granted, weighted average exercise price (in dollars per share)	0
Exercised, weighted average exercise price (in dollars per share)	12.97
Forfeited, weighted average exercise price (in dollars per share)	22.07
Options outstanding, weighted average exercise price (in dollars per share)	18.483	$ 16.172
Options exercisable at the end of the year, weighted average exercise price (in dollars per share)	$ 18.041
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, aggregate intrinsic value	$ 47,269	$ 47,152
Options outstanding, weighted average remaining contractual life (Year)	4 years 227 days	4 years 330 days
Options exercisable at the end of the year, aggregate intrinsic value	$ 43,205
Options exercisable at the end of the year, weighted average remaining contractual life (Year)	4 years 133 days