Consolidated Statements Of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning of year at Dec. 31, 2017		$ 97,523	$ 1,663,864	$ 0	$ 1,663,864
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds of stock options exercised		358
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	0	1,519,658
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds of stock options exercised		701
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		$ 1,698,660
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds of stock options exercised		0
Share-based compensation expense		142
Withholding tax on stock options exercised		0
Net income (loss)	374,607		374,607
Dividends paid to shareholders			(139,640)
End of year at Dec. 31, 2020	$ 2,032,597	$ 98,970	$ 1,933,627	$ (2,014)