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		0
Share-based compensation expense		23
Net income (loss)	$ (42,607)
Dividends paid to shareholders			(34,582)
Shareholders' equity, ending balance at Mar. 31, 2018	1,684,221	97,546	1,586,675
Shareholders' equity, beginning balance at Dec. 31, 2018	1,617,684	98,026	1,519,658
Proceeds from stock options exercised		453
Share-based compensation expense		16
Net income (loss)	135,867
Dividends paid to shareholders			(34,726)
Shareholders' equity, ending balance at Mar. 31, 2019	$ 1,719,294	$ 98,495	$ 1,620,799