Supplemental Oil and Natural Gas Reserve Data (unaudited) (Capitalized Costs) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended			2 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 28, 2015 item
Evaluated Properties
Beginning of period balance	$ 1,701,577	$ 1,497,010	$ 1,421,640
Capitalized G&A	10,071	10,014	12,148
Property acquisition costs	94,541	10,885	2,075
Exploration costs	118,251	147,164	22,703
Development costs	153,545	36,504	38,444
End of period balance	2,077,985	1,701,577	1,497,010
Unevaluated Properties
Beginning of period balance	43,222	68,776	2,603
Property acquisition costs	128,342	2,259	29,590
Exploration	11,177	10,767	34,674
Capitalized interest	4,295	4,410	2,109
Transfers to evaluated	(44,511)	(42,990)	(200)
End of period balance	142,525	43,222	68,776
Accumulated depreciation, depletion and amortization:
Beginning of period balance	1,420,612	1,296,265	1,208,331
Provision charged to expense	56,663	42,251	48,524
Sale of mineral interests	1,080	82,096	39,410
End of period balance	$ 1,478,355	$ 1,420,612	$ 1,296,265
Depletion Expense Per Physical Unit of Production	27.51	31.12	31.56
Lease Expense Per Physical Unit of Production	10.85	14.00	14.81
Subsequent Event [Member]
Accumulated depreciation, depletion and amortization:
Number of completed horizontal wells				5
Number of horizontal wells drilled				4
Number of horizontal wells in progress				2
Minimum [Member]
Accumulated depreciation, depletion and amortization:
Unevaluated Property Costs Expected Evaluation Period Range	3 years
Maximum [Member]
Accumulated depreciation, depletion and amortization:
Unevaluated Property Costs Expected Evaluation Period Range	5 years