Consolidated Statements of Shareholders' Equity Statement - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Cumulative Effect, Period Of Adoption, Adjustment [Member] Retained Earnings [Member]	Cumulative Effect, Period Of Adoption, Adjusted Balance [Member] Retained Earnings [Member]
Shareholders' equity, beginning balance at Dec. 31, 2019		$ 98,828	$ 1,700,674	$ (2,014)	$ 1,698,660
Proceeds from stock options exercised		0
Share-based compensation expense		106
Withholding tax on stock options exercised		0
Net income	$ 207,864		207,864
Dividends paid to shareholders			(104,626)
Shareholders' equity, ending balance at Sep. 30, 2020	1,900,832	98,934	1,801,898
Shareholders' equity, beginning balance at Jun. 30, 2020		98,898	1,717,917	0	1,717,917
Proceeds from stock options exercised		0
Share-based compensation expense		36
Withholding tax on stock options exercised		0
Net income	118,857		118,857
Dividends paid to shareholders			(34,876)
Shareholders' equity, ending balance at Sep. 30, 2020	1,900,832	98,934	1,801,898
Shareholders' equity, beginning balance at Dec. 31, 2020	2,032,597	98,970	1,933,627	0	1,933,627
Proceeds from stock options exercised		215
Share-based compensation expense		106
Withholding tax on stock options exercised		(383)
Net income	217,464		217,464
Dividends paid to shareholders			(105,066)
Shareholders' equity, ending balance at Sep. 30, 2021	2,144,933	98,908	2,046,025
Shareholders' equity, beginning balance at Jun. 30, 2021		98,872	2,079,759	$ 0	$ 2,079,759
Proceeds from stock options exercised		0
Share-based compensation expense		36
Withholding tax on stock options exercised		0
Net income	1,288		1,288
Dividends paid to shareholders			(35,022)
Shareholders' equity, ending balance at Sep. 30, 2021	$ 2,144,933	$ 98,908	$ 2,046,025