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. Class A 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, 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								20,017				20,017
Net change in accumulated other comprehensive income									554			554
Dividends declared Common Stock:
Dividends declared Common Stock						$ (3,891)	$ (427)			$ (3,891)	$ (427)
Stock options exercised, net of shares redeemed					33							33
Stock options exercised, net of shares redeemed (in shares)	2
Conversion of Class B Common Stock to Class A Common Stock (in shares)	2	(2)
Repurchase of Class A Common Stock			(2)		(107)			(435)				(544)
Repurchase of Class A Common Stock (in shares)	(13)
Net change in notes receivable on Class A Common Stock				$ 51						51
Deferred director compensation expense - Class A Common Stock				55						55
Deferred director compensation expense - Class A Common Stock (in shares)	5
Stock based compensation expense - performance stock units					132							132
Stock based compensation - restricted stock					215							215
Stock based compensation - restricted stock (in shares)	4
Stock based compensation - stock options					63							63
Balance at end of period at Mar. 31, 2017			4,904		138,634			475,885	1,241			620,664
Balance (in shares) at Mar. 31, 2017	18,615	2,243
Balance at beginning 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												40,794
Net change in accumulated other comprehensive income									808			808
Balance at end of period at Sep. 30, 2017			4,904		139,314			487,574	1,495			633,287
Balance (in shares) at Sep. 30, 2017	18,618	2,243
Balance at beginning 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
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
Balance at beginning of period at Mar. 31, 2017			4,904		138,634			475,885	1,241			620,664
Balance (in shares) at Mar. 31, 2017	18,615	2,243
Increase (Decrease) in Stockholders' Equity
Net income								10,071				10,071
Net change in accumulated other comprehensive income									311			311
Dividends declared Common Stock:
Dividends declared Common Stock						(4,095)	(449)			(4,095)	(449)
Net change in notes receivable on Class A Common Stock				103						103
Deferred director compensation expense - Class A Common Stock				41						41
Deferred director compensation expense - Class A Common Stock (in shares)	3
Stock based compensation expense - performance stock units					105							105
Stock based compensation - restricted stock					77							77
Stock based compensation - stock options					63							63
Balance at end of period at Jun. 30, 2017			4,904		139,023			481,412	1,552			626,891
Balance (in shares) at Jun. 30, 2017	18,618	2,243
Increase (Decrease) in Stockholders' Equity
Net income								10,706				10,706
Net change in accumulated other comprehensive income									(57)			(57)
Dividends declared Common Stock:
Dividends declared Common Stock						(4,096)	(448)			(4,096)	(448)
Repurchase of Class A Common Stock					(14)							(14)
Net change in notes receivable on Class A Common Stock				(19)						(19)
Deferred director compensation expense - Class A Common Stock				51						51
Deferred director compensation expense - Class A Common Stock (in shares)	(3)
Stock based compensation expense - performance stock units					127							127
Stock based compensation - restricted stock					81							81
Stock based compensation - restricted stock (in shares)	3
Stock based compensation - stock options					65							65
Balance at end of period at Sep. 30, 2017			4,904		139,314			487,574	1,495			633,287
Balance (in shares) at Sep. 30, 2017	18,618	2,243
Balance at beginning 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
Increase (Decrease) in Stockholders' Equity
Net income								27,469				27,469
Net change in accumulated other comprehensive income									(1,495)			(1,495)
Dividends declared Common Stock:
Dividends declared Common Stock						(4,517)	(494)			(4,517)	(494)
Net change in notes receivable on Class A Common Stock				33						33
Deferred director compensation expense - Class A Common Stock				55						55
Deferred director compensation expense - Class A Common Stock (in shares)	2
Stock based compensation expense - performance stock units					26							26
Stock based compensation - restricted stock					64							64
Stock based compensation - restricted stock (in shares)	36
Stock based compensation - stock options					62							62
Balance at end of period at Mar. 31, 2018			4,902		139,646			510,123	(1,417)			653,254
Balance (in shares) at Mar. 31, 2018	18,645	2,243
Balance at beginning 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
Increase (Decrease) in Stockholders' Equity
Net income												60,546
Net change in accumulated other comprehensive income									(2,535)			(2,535)
Balance at end of period at Sep. 30, 2018			4,904		140,834			533,191	(2,119)			676,810
Balance (in shares) at Sep. 30, 2018	18,682	2,213
Increase (Decrease) in Stockholders' Equity
Adjustment for adoption of ASU 2016-01								(35)	(338)			(373)
Balance at beginning of period at Mar. 31, 2018			4,902		139,646			510,123	(1,417)			653,254
Balance (in shares) at Mar. 31, 2018	18,645	2,243
Increase (Decrease) in Stockholders' Equity
Net income								15,666				15,666
Net change in accumulated other comprehensive income									(376)			(376)
Dividends declared Common Stock:
Dividends declared Common Stock						(4,518)	(487)			(4,518)	(487)
Stock options exercised, net of shares redeemed					48							48
Stock options exercised, net of shares redeemed (in shares)	2
Conversion of Class B Common Stock to Class A Common Stock (in shares)	28	(28)
Net change in notes receivable on Class A Common Stock				36						36
Deferred director compensation expense - Class A Common Stock	$ 1			47						48
Deferred director compensation expense - Class A Common Stock (in shares)	3
Stock based compensation expense - performance stock units					27							27
Stock based compensation - restricted stock					254							254
Stock based compensation - restricted stock (in shares)	(1)
Stock based compensation - stock options					56							56
Balance at end of period at Jun. 30, 2018			4,903		140,114			520,784	(1,793)			664,008
Balance (in shares) at Jun. 30, 2018	18,677	2,215
Increase (Decrease) in Stockholders' Equity
Net income								17,411				17,411
Net change in accumulated other comprehensive income									(326)			(326)
Dividends declared Common Stock:
Dividends declared Common Stock						$ (4,517)	$ (487)			(4,517)	$ (487)
Stock options exercised, net of shares redeemed					35							35
Stock options exercised, net of shares redeemed (in shares)	1
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				(25)						(25)
Deferred director compensation expense - Class A Common Stock				62						62
Deferred designed key employee compensation expense - Class A Common Stock				284						284
Employee stock purchase plan - Class A Common Stock	$ 1			$ 123						$ 124
Employee stock purchase plan - Class A Common Stock (in shares)	3
Stock based compensation expense - performance stock units					53							53
Stock based compensation - restricted stock					132							132
Stock based compensation - restricted stock (in shares)	(1)
Stock based compensation - stock options					56							56
Balance at end of period at Sep. 30, 2018			$ 4,904		$ 140,834			$ 533,191	$ (2,119)			$ 676,810
Balance (in shares) at Sep. 30, 2018	18,682	2,213