Condensed 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, 2016	$ 69,693	$ 72	$ 9,638	$ (3)	$ 59,986
Beginning balance, shares at Dec. 31, 2016		7,244
Common stock issued on exercise of options and vesting of restricted stock units	148	$ 1	147
Common stock issued on exercise of options and vesting of restricted stock units, shares		85
Common stock repurchased on vesting of restricted stock units	(1,666)		(1,666)
Common stock repurchased on vesting of restricted stock units, shares		(29)
Share-based compensation expense	3,064		3,064
Cash dividends on common stock	(5,449)				(5,449)
Unrealized gain (loss) on investments, net of tax	188			188
Net income	14,684				14,684
Ending balance at Sep. 30, 2017	80,662	$ 73	11,183	185	69,221
Ending balance, shares at Sep. 30, 2017		7,300
Beginning balance at Dec. 31, 2017	88,834	$ 73	12,311	(1,430)	77,880
Beginning balance, shares at Dec. 31, 2017		7,301
Common stock issued on exercise of options and vesting of restricted stock units	324	$ 1	323
Common stock issued on exercise of options and vesting of restricted stock units, shares		112
Common stock repurchased on vesting of restricted stock units	(2,952)		(2,952)
Common stock repurchased on vesting of restricted stock units, shares		(32)
Share-based compensation expense	3,992		3,992
Cash dividends on common stock	(5,499)				(5,499)
Unrealized gain (loss) on investments, net of tax	(5,976)			(5,976)
Net income	21,206				21,206
Ending balance at Sep. 30, 2018	$ 99,929	$ 74	$ 13,674	$ (7,406)	$ 93,587
Ending balance, shares at Sep. 30, 2018		7,381