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							27,469				27,469
Net change in accumulated other comprehensive income								(1,495)			(1,495)
Dividends declared on Common Stock:
Dividends declared on 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 awards, Performance stock units				26							26
Stock-based awards, Restricted stock				64							64
Stock-based awards, Restricted stock (in shares)	36
Stock-based awards, 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
Increase (Decrease) in Stockholders' Equity
Adjustment for adoption							(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							29,516				29,516
Net change in accumulated other comprehensive income								2,794			2,794
Dividends declared on Common Stock:
Dividends declared on Common Stock					$ (4,933)	$ (531)			$ (4,933)	$ (531)
Repurchase of Class A Common Stock			(1)	(376)			(2)				(379)
Repurchase of Class A Common Stock (in shares)	(8)
Net change in notes receivable on Class A Common Stock				(34)							(34)
Deferred director compensation expense - Class A Common Stock				65							65
Deferred director compensation expense - Class A Common Stock (in shares)	5
Deferred designed key employee compensation expense - Class A Common Stock				168							168
Employee stock purchase plan - Class A Common Stock				121							121
Employee stock purchase plan - Class A Common Stock (in shares)	3
Stock-based awards, Performance stock units				(57)							(57)
Stock-based awards, Performance stock units (in shares)	23
Stock-based awards, Restricted stock				180							180
Stock-based awards, Stock options				121							121
Balance at end of period at Mar. 31, 2019			$ 4,899	$ 141,206			569,189	$ 1,797			717,091
Balance (in shares) at Mar. 31, 2019	18,698	2,213
Increase (Decrease) in Stockholders' Equity
Adjustment for adoption							$ 126				$ 126