Consolidated Statements of Shareholders' Equity Statement - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]
Shareholders' equity, beginning balance at Dec. 31, 2017		$ 97,523	$ 1,663,864
Proceeds from stock options exercised		340
Share-based compensation expense		104
Withholding tax on stock options exercised		0
Net income	$ 76,151		76,151
Dividends paid to shareholders			(103,752)
Shareholders' equity, ending balance at Sep. 30, 2018	1,734,230	97,967	1,636,263
Shareholders' equity, beginning balance at Jun. 30, 2018		97,586	1,612,272
Proceeds from stock options exercised		340
Share-based compensation expense		41
Withholding tax on stock options exercised		0
Net income	58,578		58,578
Dividends paid to shareholders			(34,587)
Shareholders' equity, ending balance at Sep. 30, 2018	1,734,230	97,967	1,636,263
Shareholders' equity, beginning balance at Dec. 31, 2018	1,617,684	98,026	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
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