CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Premium on Capital Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Deferred Compensation Plan	Treasury Stock
Beginning balance, shares (in shares) at Dec. 31, 2014		75,875,711
Beginning balance, value at Dec. 31, 2014	$ 3,414,604	$ 759	$ 564,438	$ 2,997,821	$ (22,870)	$ 2,862	$ (128,406)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss)	(945,731)			(945,731)
Other comprehensive income (loss)	23,133				23,133
Purchase of treasury shares, net	(4,723)						(4,723)
Shares issued for stock offering (in shares)		5,700,000
Shares issued for stock offering	398,620	$ 57	398,563
Shares issued for employee benefit plans (in shares)		194,450
Employee benefit plans	6,739	$ 2	6,737
Deferred compensation obligation	0					(897)	897
Stock-based compensation	8,228		8,228
Tax benefit (expense) from employee stock plans	1,064		1,064
Cash dividends, per share	(6,074)			(6,074)
Ending balance, shares (in shares) at Dec. 31, 2015		81,770,161
Ending balance, value at Dec. 31, 2015	2,895,860	$ 818	979,030	2,046,016	263	1,965	(132,232)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss)	(167,513)			(167,513)
Other comprehensive income (loss)	1,142				1,142
Purchase of treasury shares, net	(2,609)						(2,609)
Shares issued for stock offering (in shares)		18,170,000
Shares issued for stock offering	381,077	$ 182	380,895
Shares issued for employee benefit plans (in shares)		198,636
Employee benefit plans	6,858	$ 1	6,857
Deferred compensation obligation	0					296	(296)
Stock-based compensation	6,043		6,043
Tax benefit (expense) from employee stock plans	$ (256)		(256)
Ending balance, shares (in shares) at Dec. 31, 2016	100,138,797	100,138,797
Ending balance, value at Dec. 31, 2016	$ 3,120,602	$ 1,001	1,372,569	1,878,503	1,405	2,261	(135,137)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of adoption of ASU No. 2016-09				(170)
Net Income (Loss)	306,828			306,828
Other comprehensive income (loss)	(1,025)				(1,025)
Purchase of treasury shares, net	(3,293)						(3,293)
Shares issued for employee benefit plans (in shares)		188,636
Employee benefit plans	9,809	$ 2	9,807
Deferred compensation obligation	0					420	(420)
Stock-based compensation	$ 5,706		5,706
Ending balance, shares (in shares) at Dec. 31, 2017	100,327,433	100,327,433
Ending balance, value at Dec. 31, 2017	$ 3,438,457	$ 1,003	$ 1,388,082	$ 2,185,161	$ 380	$ 2,681	$ (138,850)