Stockholders' Equity - Stock Options Activities (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 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)	(689,227)
Forfeited (in shares)	(1,471)
Options outstanding, Balance (in shares)	765,587	1,456,285
Options exercisable at the end of the year (in shares)	722,216
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)	13.843
Forfeited, weighted average exercise price (in dollars per share)	21.972
Options outstanding, weighted average exercise price (in dollars per share)	18.257	$ 16.172
Options exercisable at the end of the year, weighted average exercise price (in dollars per share)	$ 18.054
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Options outstanding, aggregate intrinsic value	$ 42,102	$ 47,152
Options outstanding, weighted average remaining contractual life (Year)	4 years 9 months 18 days	4 years 10 months 27 days
Options exercisable at the end of the year, aggregate intrinsic value	$ 39,900
Options exercisable at the end of the year, weighted average remaining contractual life (Year)	4 years 8 months 6 days