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, 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				60,546
Net change in accumulated other comprehensive income								(2,197)			(2,197)
Dividends declared on Common Stock:
Dividends declared on Common Stock					$ (13,552)	$ (1,468)			$ (13,552)	$ (1,468)
Stock options exercised, net of shares withheld				83							83
Stock options exercised, net of shares withheld (in shares)	3
Conversion of Class B Common Stock to Class A Common Stock (in shares)	30	(30)
Net change in notes receivable on Class A Common Stock				44							44
Deferred director compensation expense - Class A Common Stock			1	164							165
Deferred director compensation expense - Class A Common Stock (in shares)	5
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 awards, Performance stock units				106							106
Stock-based awards, Restricted stock				450							450
Stock-based awards, Restricted stock (in shares)	34
Stock-based awards, Stock options				174							174
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
Balance at beginning 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 on Common Stock:
Dividends declared on Common Stock					(4,517)	(487)			(4,517)	(487)
Stock options exercised, net of shares withheld				35							35
Stock options exercised, net of shares withheld (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 awards, Performance stock units				53							53
Stock-based awards, Performance stock units (in shares)	(1)
Stock-based awards, Restricted stock				132							132
Stock-based awards, 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
Increase (Decrease) in Stockholders' Equity
Adjustment for adoption | ASU 2016-01							(35)	(338)			(373)
Balance at beginning of period at Dec. 31, 2018			4,900	141,018			545,013	(997)			689,934
Balance (in shares) at Dec. 31, 2018	18,675	2,213
Increase (Decrease) in Stockholders' Equity
Net income							65,931				65,931
Net change in accumulated other comprehensive income								4,153			4,153
Dividends declared on Common Stock:
Dividends declared on Common Stock					(14,826)	(1,591)			(14,826)	(1,591)
Stock options exercised, net of shares withheld			9	(183)							(174)
Stock options exercised, net of shares withheld (in shares)	37
Conversion of Class B Common Stock to Class A Common Stock (in shares)	5	(5)
Repurchase of Class A Common Stock			(2)	(404)			(174)				(580)
Repurchase of Class A Common Stock (in shares)	(12)
Net change in notes receivable on Class A Common Stock				(238)							(238)
Deferred director compensation expense - Class A Common Stock				155							155
Deferred director compensation expense - Class A Common Stock (in shares)	5
Deferred designed key employee compensation expense - Class A Common Stock				302							302
Employee stock purchase plan - Class A Common Stock				372							372
Employee stock purchase plan - Class A Common Stock (in shares)	8
Stock-based awards, Performance stock units				(57)							(57)
Stock-based awards, Performance stock units (in shares)	23
Stock-based awards, Restricted stock			1	659							660
Stock-based awards, Restricted stock (in shares)	3
Stock-based awards, Stock options				280							280
Balance at end of period at Sep. 30, 2019			4,908	141,904			594,479	3,156			744,447
Balance (in shares) at Sep. 30, 2019	18,744	2,208
Balance at beginning of period at Jun. 30, 2019			4,907	141,525			581,734	3,262			731,428
Balance (in shares) at Jun. 30, 2019	18,740	2,208
Increase (Decrease) in Stockholders' Equity
Net income							18,408				18,408
Net change in accumulated other comprehensive income								(106)			(106)
Dividends declared on Common Stock:
Dividends declared on Common Stock					$ (4,961)	$ (530)			$ (4,961)	$ (530)
Stock options exercised, net of shares withheld			1	(73)							(72)
Stock options exercised, net of shares withheld (in shares)	3
Repurchase of Class A Common Stock			(1)	(28)			(172)				(201)
Repurchase of Class A Common Stock (in shares)	(4)
Net change in notes receivable on Class A Common Stock				(46)							(46)
Deferred director compensation expense - Class A Common Stock				49							49
Deferred designed key employee compensation expense - Class A Common Stock				76							76
Employee stock purchase plan - Class A Common Stock				137							137
Employee stock purchase plan - Class A Common Stock (in shares)	3
Stock-based awards, Restricted stock			1	184							185
Stock-based awards, Restricted stock (in shares)	2
Stock-based awards, Stock options				80							80
Balance at end of period at Sep. 30, 2019			$ 4,908	$ 141,904			594,479	$ 3,156			744,447
Balance (in shares) at Sep. 30, 2019	18,744	2,208
Increase (Decrease) in Stockholders' Equity
Adjustment for adoption | ASU 2016-02							$ 126				$ 126