Assets Held for Sale (Details) (USD $)	9 Months Ended						9 Months Ended	3 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 24, 2014 Gooseneck Prospect Acquisition [Member]	Sep. 24, 2014 Gooseneck Prospect Acquisition [Member] Fair Value, Measurements, Nonrecurring [Member]	Oct. 15, 2014 Gooseneck Prospect Acquisition [Member] Subsequent Event [Member]	Sep. 30, 2014 Rocky Mountain [Member]	Jun. 30, 2014 Rocky Mountain [Member]
Business Acquisition [Line Items]
Net acres acquired				61,000
Cash Consideration to Acquire Proved and Unproved Property				$ 325,230,000		$ 84,800,000
Proved Oil and Gas Property, Successful Effort Method	6,789,927,000		5,637,462,000		201,842,000
Unproved Oil and Gas Property, Successful Effort Method	536,100,000		271,100,000		125,562,000
Oil and Gas Property, Successful Effort Method, Net					327,404,000
Working Capital Acquired					(61,000)
Asset Retirement Obligations, Noncurrent	123,905,000		115,659,000		(2,113,000)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net					325,230,000
Leasehold Acres Consideration to Acquire Unproved Property							7,000
Payments to Acquire Oil and Gas Property							134,100,000
Proceeds from Sale of Oil and Gas Property and Equipment	41,868,000	20,498,000						50,200,000
Gain (Loss) on Disposition of Oil and Gas Property								26,800,000
Assets Held-for-sale Reasonably Certain Period for Sale	1
Disposal Group, Including Discontinued Operation, Long Lived Assets, Current	19,900,000
Asset retirement obligation associated with oil and gas properties held for sale	$ 452,000		$ 3,033,000