CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Treasury Stock	Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Balance at beginning of period at Dec. 31, 2014	$ 2,142,733	$ 42,292	$ (306,835)	$ 710,432	$ (2,151)	$ 1,698,995
Balance (in shares) at Dec. 31, 2014		422,915	9,893
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(113,891)					(113,891)
Issuance of common stock	(942)	$ 4		(946)
Exercise of stock appreciation rights (in shares)		40
Exercise of stock appreciation rights	12,592	$ 81		12,511
Stock amortization and vesting (in shares)		814
Cash dividends	(33,090)					(33,090)
Other comprehensive income	1,786				1,786
Balance at end of period at Dec. 31, 2015	2,009,188	$ 42,377	$ (306,835)	721,997	(365)	1,552,014
Balance (in shares) at Dec. 31, 2015		423,769	9,893
Increase (Decrease) in Stockholders' Equity
Net income (loss)	(417,124)					(417,124)
Issuance of common stock (in shares)		50,600
Issuance of common stock	995,289	$ 5,060		990,229
Issuance of common stock	(198)	$ 3		(201)
Exercise of stock appreciation rights (in shares)		28
Exercise of stock appreciation rights	16,931	$ 64		16,867
Sale of stock held in rabbi trust	544			544
Stock amortization and vesting	(2,126)			(2,126)
Stock amortization and vesting (in shares)		646
Cash dividends	(36,187)					(36,187)
Other comprehensive income	1,350				1,350
Balance at end of period at Dec. 31, 2016	2,567,667	$ 47,504	$ (306,835)	1,727,310	985	1,098,703
Balance (in shares) at Dec. 31, 2016		475,043	9,893
Increase (Decrease) in Stockholders' Equity
Cumulative effect from accounting change	42,172					42,172
Net income (loss)	100,393					100,393
Issuance of common stock	0	$ 14		(14)
Exercise of stock appreciation rights (in shares)		137
Exercise of stock appreciation rights	15,160	$ 37		15,123
Stock amortization and vesting (in shares)		367
Cash dividends	(78,838)					(78,838)
Purchase of treasury stock (in shares)			5,043
Purchase of treasury stock	(123,741)		$ (123,741)
Other comprehensive income	1,092				1,092
Balance at end of period at Dec. 31, 2017	$ 2,523,905	$ 47,555	$ (430,576)	$ 1,742,419	$ 2,077	$ 1,162,430
Balance (in shares) at Dec. 31, 2017		475,547	14,936