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, 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	995,289	$ 5,060		990,229
Issuance of common stock (in shares)		50,600
Exercise of stock appreciation rights	(198)	$ 3		(201)
Exercise of stock appreciation rights (in shares)		28
Stock amortization and vesting	16,931	$ 64		16,867
Stock amortization and vesting (in shares)		646
Sale of stock held in rabbi trust	544			544
Stock-based compensation	(2,126)			(2,126)
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 impact from accounting change	42,172					42,172
Net income (loss)	100,393					100,393
Exercise of stock appreciation rights	0	$ 14		(14)
Exercise of stock appreciation rights (in shares)		137
Stock amortization and vesting	15,160	$ 37		15,123
Stock amortization and vesting (in shares)		367
Cash dividends	(78,838)					(78,838)
Purchase of treasury stock	(123,741)		$ (123,741)
Purchase of treasury stock (in shares)			5,043
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
Increase (Decrease) in Stockholders' Equity
Net income (loss)	557,043					557,043
Exercise of stock appreciation rights	0	$ 1		(1)
Exercise of stock appreciation rights (in shares)		9
Stock amortization and vesting	20,778	$ 54		20,724
Stock amortization and vesting (in shares)		539
Cash dividends	(111,369)					(111,369)
Purchase of treasury stock	(904,112)		$ (904,112)
Purchase of treasury stock (in shares)			38,474
Other comprehensive income	2,360				2,360
Cumulative impact from accounting change	(446)					(446)
Balance at end of period at Dec. 31, 2018	$ 2,088,159	$ 47,610	$ (1,334,688)	$ 1,763,142	$ 4,437	$ 1,607,658
Balance (in shares) at Dec. 31, 2018		476,095	53,410