Acquisitions, Divestitures, and Assets Held for Sale (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Business Acquisition [Line Items]
Proved Oil and Gas Property, Successful Effort Method	$ 7,006,832,000		$ 7,348,436,000
Unproved Oil and Gas Property, Successful Effort Method	512,461,000		532,498,000
Asset Retirement Obligations, Noncurrent	14,286,000		438,000
Proceeds from Sale of Oil and Gas Property and Equipment	21,573,000	1,979,000
Assets Held-for-sale Reasonably Certain Period for Sale	1
Document Period End Date	Mar. 31, 2015
Disposal Group, Including Discontinued Operation, Long Lived Assets, Current	184,951,000		17,891,000
Mid Continent Divestiture 2015 [Member]
Business Acquisition [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal	30,000,000
Significant Acquisitions and Disposals, Acquisition Costs or Sale Proceeds	324,000,000
Total Expected Exit and Disposal Costs	10,000,000
Business Exit Costs	$ 3,500,000