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									$ 687
Increase (Decrease) in Stockholders' Equity
Net income												$ 20,017
Balance at end of period at Mar. 31, 2017									1,241
Increase (Decrease) in Stockholders' Equity
Adjustment for adoption of ASU 2016-01 | ASU 2016-01								$ (35)	(338)			(373)
Balance at beginning of period, as adjusted		$ 2,243	$ 4,902		$ 139,406			487,665	78			632,051
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