Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive (Loss) Income [Member]	Retained Earnings [Member]
Beginning Balance at Dec. 31, 2014	$ 32,820	$ 71	$ 4,410	$ (23)	$ 28,362
Beginning Balance, shares at Dec. 31, 2014		7,126
Common stock issued on exercise of options and vesting of restricted stock units	702	$ 1	701
Common stock issued on exercise of options and vesting of restricted stock units, shares		89
Common stock repurchased on vesting of restricted stock units	(465)		(465)
Common stock repurchased on vesting of restricted stock units, shares		(12)
Share-based compensation expense	2,386		2,386
Excess tax benefits from share-based compensation	(68)		(68)
Cash dividends on common stock	(6,310)				(6,310)
Unrealized (losses) gains on investments, net of tax	(8)			(8)
Net income	25,494				25,494
Ending Balance at Dec. 31, 2015	54,551	$ 72	6,964	(31)	47,546
Ending Balance, shares at Dec. 31, 2015		7,203
Common stock issued on exercise of options and vesting of restricted stock units	72		72
Common stock issued on exercise of options and vesting of restricted stock units, shares		52
Common stock repurchased on vesting of restricted stock units	(433)		(433)
Common stock repurchased on vesting of restricted stock units, shares		(11)
Share-based compensation expense	2,782		2,782
Excess tax benefits from share-based compensation	253		253
Cash dividends on common stock	(6,359)				(6,359)
Unrealized (losses) gains on investments, net of tax	28			28
Net income	18,799				18,799
Ending Balance at Dec. 31, 2016	69,693	$ 72	9,638	(3)	59,986
Ending Balance, shares at Dec. 31, 2016		7,244
Common stock issued on exercise of options and vesting of restricted stock units	163	$ 1	162
Common stock issued on exercise of options and vesting of restricted stock units, shares		86
Common stock repurchased on vesting of restricted stock units	(1,673)		(1,673)
Common stock repurchased on vesting of restricted stock units, shares		(29)
Share-based compensation expense	4,184		4,184
Cash dividends on common stock	(7,276)				(7,276)
Unrealized (losses) gains on investments, net of tax	(1,427)			(1,427)
Net income	25,170				25,170
Ending Balance at Dec. 31, 2017	$ 88,834	$ 73	$ 12,311	$ (1,430)	$ 77,880
Ending Balance, shares at Dec. 31, 2017		7,301