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		88
Withholding tax on stock options exercised		(22)
Net income	$ 288,399		288,399
Dividends paid to shareholders			(104,196)
Shareholders' equity, ending balance at Sep. 30, 2019	1,802,544	98,683	1,703,861
Shareholders' equity, beginning balance at Jun. 30, 2019		98,647	1,669,315	0	1,669,315
Proceeds from stock options exercised		0
Share-based compensation expense		36
Withholding tax on stock options exercised		0
Net income	69,282		69,282
Dividends paid to shareholders			(34,736)
Shareholders' equity, ending balance at Sep. 30, 2019	1,802,544	98,683	1,703,861
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		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