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 at Dec. 31, 2012		75,067,760
Beginning balance, value at Dec. 31, 2012	$ 2,676,690	$ 751	$ 494,910	$ 2,314,055	$ (8,311)	$ 2,774	$ (127,489)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	204,554			204,554
Other comprehensive income (loss)	(11,756)				(11,756)
Purchase of treasury shares, net (73,206 shares in 2015, 32,768 shares in 2014, 14,766 shares in 2013)	(1,038)						(1,038)
Purchase and retirement of shares, shares		0
Shares issued for employee benefit plans, shares		506,396
Shares issued for employee benefit plans	18,795	$ 5	18,790
Deferred compensation obligation	0					485	(485)
Stock-based compensation	9,625		6,869				2,756
Tax benefit from employee stock plans	3,142		3,142
Cash dividends, per share - ($0.08 in 2015, $0.47 in 2014, $0.58 in 2013)	(41,993)			(41,993)
Ending balance, shares at Dec. 31, 2013		75,574,156
Ending balance, value at Dec. 31, 2013	2,858,019	$ 756	523,711	2,476,616	(20,067)	3,259	(126,256)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	568,032			568,032
Other comprehensive income (loss)	(2,803)				(2,803)
Purchase of treasury shares, net (73,206 shares in 2015, 32,768 shares in 2014, 14,766 shares in 2013)	(2,547)						(2,547)
Purchase and retirement of shares, shares		(226,839)
Purchase and retirement of shares, value	(14,913)	$ (2)	(2,388)	(12,523)
Shares issued for employee benefit plans, shares		528,394
Shares issued for employee benefit plans	25,501	$ 5	25,496
Deferred compensation obligation	0					(397)	397
Stock-based compensation	11,713		11,713
Tax benefit from employee stock plans	5,906		5,906
Cash dividends, per share - ($0.08 in 2015, $0.47 in 2014, $0.58 in 2013)	$ (34,304)			(34,304)
Ending balance, shares at Dec. 31, 2014	75,875,711	75,875,711
Ending 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 (73,206 shares in 2015, 32,768 shares in 2014, 14,766 shares in 2013)	(4,723)						(4,723)
Shares issued for stock offering, shares		5,700,000
Shares issued for stock offering	398,620	$ 57	398,563
Purchase and retirement of shares, shares		0
Shares issued for employee benefit plans, shares		194,450
Shares issued for employee benefit plans	6,739	$ 2	6,737
Deferred compensation obligation	0					(897)	897
Stock-based compensation	8,228		8,228
Tax benefit from employee stock plans	1,064		1,064
Cash dividends, per share - ($0.08 in 2015, $0.47 in 2014, $0.58 in 2013)	$ (6,074)			(6,074)
Ending balance, shares at Dec. 31, 2015	81,770,161	81,770,161
Ending balance, value at Dec. 31, 2015	$ 2,895,860	$ 818	$ 979,030	$ 2,046,016	$ 263	$ 1,965	$ (132,232)