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		70
Withholding tax on stock options exercised		0
Net income	$ 89,007		89,007
Dividends paid to shareholders			(69,750)
Shareholders' equity, ending balance at Jun. 30, 2020	1,816,815	98,898	1,717,917
Shareholders' equity, beginning balance at Mar. 31, 2020		98,863	1,524,581	0	1,524,581
Proceeds from stock options exercised		0
Share-based compensation expense		35
Withholding tax on stock options exercised		0
Net income	228,211		228,211
Dividends paid to shareholders			(34,875)
Shareholders' equity, ending balance at Jun. 30, 2020	1,816,815	98,898	1,717,917
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		70
Withholding tax on stock options exercised		(383)
Net income	216,176		216,176
Dividends paid to shareholders			(70,044)
Shareholders' equity, ending balance at Jun. 30, 2021	2,178,631	98,872	2,079,759
Shareholders' equity, beginning balance at Mar. 31, 2021		98,837	2,005,600	$ 0	$ 2,005,600
Proceeds from stock options exercised		0
Share-based compensation expense		35
Withholding tax on stock options exercised		0
Net income	109,181		109,181
Dividends paid to shareholders			(35,022)
Shareholders' equity, ending balance at Jun. 30, 2021	$ 2,178,631	$ 98,872	$ 2,079,759