CONSOLIDATED STATEMENTS OF STOCKHOLDERS EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Retained Earnings	Treasury Stock	Total
Balance at Dec. 31, 2017	$ 191	$ 26,890	$ 257,516	$ (18,123)	$ 266,474
Balance (in shares) at Dec. 31, 2017	17,759,446
Increase (Decrease) in Stockholders' Equity
Net exercise of stock options		85		465	550
Net exercise of stock options (in shares)	32,938
Stock-based compensation expense		566			566
Adjustment to beginning retained earnings for accounting changes in accordance with the new revenue recognition standard			(4,858)		(4,858)
Net income			6,741		6,741
Balance at Mar. 31, 2018	$ 191	27,541	259,399	(17,658)	269,473
Balance (in shares) at Mar. 31, 2018	17,792,384
Balance at Dec. 31, 2017	$ 191	26,890	257,516	(18,123)	266,474
Balance (in shares) at Dec. 31, 2017	17,759,446
Increase (Decrease) in Stockholders' Equity
Net income					26,839
Balance at Sep. 30, 2018	$ 191	29,458	279,497	(16,494)	292,652
Balance (in shares) at Sep. 30, 2018	17,894,057
Balance at Mar. 31, 2018	$ 191	27,541	259,399	(17,658)	269,473
Balance (in shares) at Mar. 31, 2018	17,792,384
Increase (Decrease) in Stockholders' Equity
Net exercise of stock options		(86)		1,157	1,071
Net exercise of stock options (in shares)	82,269
Stock-based compensation expense		825			825
Net income			9,239		9,239
Balance at Jun. 30, 2018	$ 191	28,280	268,638	(16,501)	280,608
Balance (in shares) at Jun. 30, 2018	17,874,653
Increase (Decrease) in Stockholders' Equity
Net exercise of stock options		276		7	283
Net exercise of stock options (in shares)	19,404
Stock-based compensation expense		902			902
Net income			10,859		10,859
Balance at Sep. 30, 2018	$ 191	29,458	279,497	(16,494)	292,652
Balance (in shares) at Sep. 30, 2018	17,894,057
Balance at Dec. 31, 2018	$ 191	30,111	286,756	(15,876)	$ 301,182
Balance (in shares) at Dec. 31, 2018	17,919,021				17,919,021
Increase (Decrease) in Stockholders' Equity
Net exercise of stock options		241		804	$ 1,045
Net exercise of stock options (in shares)	57,670
Stock-based compensation expense		915			915
Net income			7,015		7,015
Balance at Mar. 31, 2019	$ 191	31,267	293,771	(15,072)	310,157
Balance (in shares) at Mar. 31, 2019	17,976,691
Balance at Dec. 31, 2018	$ 191	30,111	286,756	(15,876)	$ 301,182
Balance (in shares) at Dec. 31, 2018	17,919,021				17,919,021
Increase (Decrease) in Stockholders' Equity
Net income					$ 25,620
Balance at Sep. 30, 2019	$ 191	33,728	312,376	(13,602)	$ 332,693
Balance (in shares) at Sep. 30, 2019	18,082,135				18,082,135
Balance at Mar. 31, 2019	$ 191	31,267	293,771	(15,072)	$ 310,157
Balance (in shares) at Mar. 31, 2019	17,976,691
Increase (Decrease) in Stockholders' Equity
Net exercise of stock options		141		545	686
Net exercise of stock options (in shares)	39,043
Stock-based compensation expense		1,003			1,003
Net income			9,279		9,279
Balance at Jun. 30, 2019	$ 191	32,411	303,050	(14,527)	321,125
Balance (in shares) at Jun. 30, 2019	18,015,734
Increase (Decrease) in Stockholders' Equity
Net exercise of stock options		290		925	1,215
Net exercise of stock options (in shares)	66,401
Stock-based compensation expense		1,027			1,027
Net income			9,326		9,326
Balance at Sep. 30, 2019	$ 191	$ 33,728	$ 312,376	$ (13,602)	$ 332,693
Balance (in shares) at Sep. 30, 2019	18,082,135				18,082,135