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, 2018		$ 98,026	$ 1,519,658	$ 0	$ 1,519,658
Proceeds from stock options exercised		591
Share-based compensation expense		52
Withholding tax on stock options exercised		(22)
Net income	$ 219,117		219,117
Dividends paid to shareholders			(69,460)
Shareholders' equity, ending balance at Jun. 30, 2019	1,767,962	98,647	1,669,315
Shareholders' equity, beginning balance at Mar. 31, 2019		98,495	1,620,799	0	1,620,799
Proceeds from stock options exercised		138
Share-based compensation expense		36
Withholding tax on stock options exercised		(22)
Net income	83,250		83,250
Dividends paid to shareholders			(34,734)
Shareholders' equity, ending balance at Jun. 30, 2019	1,767,962	98,647	1,669,315
Shareholders' equity, beginning balance at Dec. 31, 2019	1,799,502	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