Consolidated Statements Of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]
Beginning of year at Dec. 31, 2016		$ 95,529	$ 1,656,873
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds of stock options exercised		2,102
Reclassification of restricted stock units from equity to liability award		(168)
Share-based compensation expense		60
Withholding tax on stock options exercised		0
Net income (loss)	$ 144,877		144,877
Dividends paid to shareholders			(137,886)
End of year at Dec. 31, 2017	1,761,387	97,523	1,663,864
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds of stock options exercised		358
Reclassification of restricted stock units from equity to liability award		0
Share-based compensation expense		145
Withholding tax on stock options exercised		0
Net income (loss)	(5,728)		(5,728)
Dividends paid to shareholders			(138,478)
End of year at Dec. 31, 2018	1,617,684	98,026	1,519,658
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds of stock options exercised		701
Reclassification of restricted stock units from equity to liability award		0
Share-based compensation expense		123
Withholding tax on stock options exercised		(22)
Net income (loss)	320,087		320,087
Dividends paid to shareholders			(139,071)
End of year at Dec. 31, 2019	$ 1,799,502	$ 98,828	$ 1,700,674