Condensed Consolidated Statement of Stockholders' Equity (Unaudited) - 9 months ended Sep. 30, 2018 - USD ($) $ in Thousands	Common Stock [Member] Common Class A [Member]	Common Stock [Member]	Retained Earnings [Member]	Treasury Stock [Member] Common Class A [Member]	Treasury Stock [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2017	8,349,069	88,788,664		(1,750,692)	(5,535,076)
Balance at Dec. 31, 2017	$ 24,644	$ 902,518	$ 161,694	$ (24,232)	$ (49,562)	$ (22,165)	$ 992,897
Net income			122,536				122,536
Restricted stock awards (in shares)	220,080	391,836		(89,422)	(107,456)
Restricted stock awards				$ (1,252)	$ (1,757)		(3,009)
Restricted stock unit awards (in shares)		209,500			(82,201)
Restricted stock unit awards					$ (1,344)		(1,344)
Forfeiture of restricted stock awards (in shares)		(91,057)
Forfeitures		$ (528)					(528)
Repurchase of common stock (in shares)					(1,551,710)
Repurchase of common stock					$ (19,607)		(19,607)
Share-based compensation (in shares)
Share-based compensation	$ 1,530	$ 4,015					5,545
Balance (in shares) at Sep. 30, 2018	8,569,149	89,298,943		(1,840,114)	(7,276,443)
Balance at Sep. 30, 2018	$ 26,174	$ 906,005	$ 284,230	$ (25,484)	$ (72,270)	$ (22,165)	$ 1,096,490