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		453
Share-based compensation expense		16
Net income	$ 135,867		135,867
Dividends paid to shareholders			(34,726)
Shareholders' equity, ending balance at Mar. 31, 2019	1,719,294	98,495	1,620,799
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		35
Net income	(139,204)		(139,204)
Dividends paid to shareholders			(34,875)
Shareholders' equity, ending balance at Mar. 31, 2020	$ 1,623,444	$ 98,863	$ 1,524,581