Accounting Changes and Error Corrections Accounting Changes and Error Corrections (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended			6 Months Ended			12 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Statement of Financial Position [Abstract]
Accounts receivable	$ 69,387			$ 69,387			$ 70,897
Deferred tax asset	9,795			9,795			10,715
Other current assets	21,032			21,032			7,600
Total Current Assets	100,778			100,778			92,489
Accumulated depreciation, depletion, and amortization	(3,261,672)			(3,261,672)			(3,125,282)
Property and Equipment, Net	2,661,785			2,661,785
Total Assets	2,777,593			2,777,593			2,698,505
Deferred Tax Liabilities	251,482			251,482			241,205
Retained earnings	327,513			327,513			315,244
Total Stockholders' Equity	1,085,145			1,085,145			1,065,350
Total Liabilities and Stockholders' Equity	2,777,593			2,777,593			2,698,505
Income Statement [Abstract]
Oil and gas sales	158,487		142,503	307,544		289,034
Total Revenues	155,994		144,077	300,174		290,368
General and administrative, net	12,058		11,191	22,584		23,916
Depreciation, Depletion and Amortization	73,090		59,773	135,741		120,229
Lease operating cost	23,572		26,957	48,539		53,841
Total Costs and Expenses	144,233		131,766	276,706		265,171
Transportation and gas processing	6,013		4,865	11,305		9,603
Severance and Other Taxes	9,436		10,501	18,638		20,526
Income Before Income Taxes	11,761		12,311	23,468		25,197
Provision for Income Taxes	4,934		4,762	11,199		9,610
Net Income	6,827		7,549	12,269		15,587	(2,442)
Earnings Per Share, Basic	$ 0.16		$ 0.17	$ 0.28		$ 0.36
Earnings Per Share, Diluted	$ 0.15		$ 0.17	$ 0.28		$ 0.36
Statement of Cash Flows [Abstract]
Deferred income taxes				11,199		9,610
Stock-based compensation expenses				3,683		6,018
Other Noncash Income (Expense)				(2,439)		(6,321)
(Increase) Decrease in accounts receivable				1,360		(1,064)
Increase (decrease) in accounts payable and accrued liabilities				5,895		2,320
Net Cash Provided by Operating Activities				170,303		149,531
Scenario, Previously Reported [Member]
Statement of Financial Position [Abstract]
Accounts receivable	68,615			68,615
Deferred tax asset	4,829			4,829
Other current assets	21,033			21,033
Total Current Assets	95,041			95,041
Accumulated depreciation, depletion, and amortization	(3,308,992)			(3,308,992)
Property and Equipment, Net	2,614,465			2,614,465
Total Assets	2,724,536			2,724,536
Deferred Tax Liabilities	229,107			229,107
Retained earnings	296,831			296,831
Total Stockholders' Equity	1,054,463			1,054,463
Total Liabilities and Stockholders' Equity	2,724,536			2,724,536
Income Statement [Abstract]
Oil and gas sales	158,214		140,892	306,772		287,369
Total Revenues	155,721		142,466	299,402		288,703
General and administrative, net	12,412		11,191	23,151		23,916
Depreciation, Depletion and Amortization	72,205		59,458	133,890		119,578
Lease operating cost	21,932		26,957	47,199		54,381
Total Costs and Expenses	142,062		131,451	274,082		266,102
Transportation and gas processing				11,305		10,895
Severance and Other Taxes				18,638		20,276
Income Before Income Taxes	13,659		11,015	25,320		22,601
Provision for Income Taxes	5,621		4,293	11,869		8,670
Net Income	8,038		6,722	13,451		13,931
Earnings Per Share, Basic	$ 0.18		$ 0.15	$ 0.31		$ 0.32
Earnings Per Share, Diluted	$ 0.18		$ 0.15	$ 0.30		$ 0.32
Statement of Cash Flows [Abstract]
Deferred income taxes				11,869		8,670
Stock-based compensation expenses				3,953		6,018
Other Noncash Income (Expense)				(2,439)		(6,024)
(Increase) Decrease in accounts receivable				2,132		601
Increase (decrease) in accounts payable and accrued liabilities				4,852		3,605
Net Cash Provided by Operating Activities				170,303		149,531
Restatement Adjustment [Member]
Statement of Financial Position [Abstract]
Accounts receivable	772			772
Deferred tax asset	4,966			4,966
Other current assets	(1)			(1)
Total Current Assets	5,737			5,737
Accumulated depreciation, depletion, and amortization	47,320	[1]		47,320	[1]
Property and Equipment, Net	47,320			47,320
Total Assets	53,057			53,057
Deferred Tax Liabilities	22,375	[1]		22,375	[1]
Retained earnings	30,682	[1]		30,682	[1]
Total Stockholders' Equity	30,682			30,682
Total Liabilities and Stockholders' Equity	53,057			53,057
Income Statement [Abstract]
Oil and gas sales	273		1,611	772		1,665
Total Revenues	273		1,611	772		1,665
General and administrative, net	(354)		0	(567)		0
Depreciation, Depletion and Amortization	885		315	1,851		651
Lease operating cost	1,640		0	1,340		(540)
Total Costs and Expenses	2,171		315	2,624		(931)
Transportation and gas processing				0		(1,292)
Severance and Other Taxes				0		250
Income Before Income Taxes	(1,898)		1,296	(1,852)		2,596
Provision for Income Taxes	(687)		469	(670)		940
Net Income	(1,211)		827	(1,182)		1,656
Earnings Per Share, Basic	$ (0.02)		$ 0.02	$ (0.03)		$ 0.04
Earnings Per Share, Diluted	$ (0.03)		$ 0.02	$ (0.02)		$ 0.04
Statement of Cash Flows [Abstract]
Deferred income taxes				(670)		940
Stock-based compensation expenses				(270)		0
Other Noncash Income (Expense)				0		(297)
(Increase) Decrease in accounts receivable				(772)		(1,665)
Increase (decrease) in accounts payable and accrued liabilities				1,043		(1,285)
Net Cash Provided by Operating Activities				$ 0		$ 0

[1] The adjustments column includes the impact of correcting the error in the ceiling test write-down as of June 30, 2014. Included in these amounts are approximately $48 million related to Accumulated depreciation, depletion, and amortization, $18 million related to the net deferred tax liabilities and $30 million related to retained earnings.