Acquisitions and Divestitures 2014 Acquisitions (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 23, 2014	Feb. 25, 2014 acre	Sep. 06, 2013		Jul. 12, 2012	Jun. 11, 2014
Significant Acquisitions and Disposals [Line Items]
Payments to acquire mineral rights	$ 7,305,000	$ 0	$ 0
Midland Basin
Significant Acquisitions and Disposals [Line Items]
Area of Land (in acres)	8,156
Payments to acquire property, plant, and equipment	192,500,000
Proved and Unproved Oil and Natural Gas Properties in Glasscock County Texas
Significant Acquisitions and Disposals [Line Items]
Area of Land (in acres)				24	278
Cash consideration				1,800,000		33,710,000	[1]	20,496,000
Payments to acquire mineral rights					7,300,000
Proved and Unproved Oil and Natural Gas Properties in Reagan County Texas
Significant Acquisitions and Disposals [Line Items]
Area of Land (in acres)									460
Cash consideration									$ 4,700,000

[1] The fair value of the acquired assets and liabilities were allocated in the following manner: $9.7 million to evaluated properties, $27.1 million to unevaluated properties, $0.2 million to other assets and $0.2 million to other liabilities.