CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Common Stock	Additional Paid In Capital.	Retained Earnings Class A Common Stock	Retained Earnings Class B Common Stock	Retained Earnings	Accumulated Other Comprehensive Income	Class A Common Stock	Class B Common Stock	Total
Balance at beginning of period at Dec. 31, 2014			$ 4,904	$ 134,889			$ 414,623	$ 4,315			$ 558,731
Balance (in shares) at Dec. 31, 2014	18,603	2,245
Increase (Decrease) in Stockholders' Equity
Net income							35,166				35,166
Net change in accumulated other comprehensive income								(2,266)			(2,266)
Dividends declared Common Stock:
Dividends declared Common Stock					$ (14,531)	$ (1,594)			$ (14,531)	$ (1,594)
Stock options exercised, net of shares redeemed			16	1,708			(588)				1,136
Stock options exercised, net of shares redeemed (in shares)	67
Repurchase of Class A Common Stock			(5)	(143)			(403)				(551)
Repurchase of Class A Common Stock (in shares)	(22)
Net change in notes receivable on Class A Common Stock				(189)							(189)
Deferred director compensation expense - Class A Common Stock				223							223
Deferred director compensation expense - Class A Common Stock (in shares)	5
Stock based compensation - restricted stock				253							253
Stock based compensation - restricted stock (in shares)	(1)
Stock based compensation expense - options				169							169
Balance at end of period at Dec. 31, 2015			4,915	136,910			432,673	2,049			576,547
Balance (in shares) at Dec. 31, 2015	18,652	2,245
Increase (Decrease) in Stockholders' Equity
Net income							45,903				45,903
Net change in accumulated other comprehensive income								(1,362)			(1,362)
Dividends declared Common Stock:
Dividends declared Common Stock					(15,359)	(1,685)			(15,359)	(1,685)
Stock options exercised, net of shares redeemed				80							80
Stock options exercised, net of shares redeemed (in shares)	4
Repurchase of Class A Common Stock			(9)	(287)			(911)				(1,207)
Repurchase of Class A Common Stock (in shares)	(43)
Net change in notes receivable on Class A Common Stock				289							289
Deferred director compensation expense - Class A Common Stock				170							170
Deferred director compensation expense - Class A Common Stock (in shares)	4
Stock based compensation expense - performance stock units				524							524
Stock based compensation - restricted stock				258							258
Stock based compensation - restricted stock (in shares)	(2)
Stock based compensation expense - options				248							248
Balance at end of period at Dec. 31, 2016			4,906	138,192			460,621	687			604,406
Balance (in shares) at Dec. 31, 2016	18,615	2,245
Increase (Decrease) in Stockholders' Equity
Net income							45,632				45,632
Net change in accumulated other comprehensive income								(271)			(271)
Dividends declared Common Stock:
Dividends declared Common Stock					$ (16,158)	$ (1,773)			$ (16,158)	$ (1,773)
Stock options exercised, net of shares redeemed				68							68
Stock options exercised, net of shares redeemed (in shares)	4
Repurchase of Class A Common Stock			(4)	(422)			(622)				(1,048)
Repurchase of Class A Common Stock (in shares)	(26)
Conversion of Class B Common Stock to Class A Common Stock (in shares)	2	(2)
Net change in notes receivable on Class A Common Stock				235							235
Deferred director compensation expense - Class A Common Stock				191							191
Deferred director compensation expense - Class A Common Stock (in shares)	5
Stock based compensation expense - performance stock units				491							491
Stock based compensation - restricted stock				424							424
Stock based compensation - restricted stock (in shares)	7
Stock based compensation expense - options				227							227
Balance at end of period at Dec. 31, 2017			$ 4,902	$ 139,406			$ 487,700	$ 416			$ 632,424
Balance (in shares) at Dec. 31, 2017	18,607	2,243