Fair Value Measurements Fair Value Measurements (Fair Value of Liabilities Measured on Nonrecurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended			3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2013 Southern Portion of Midland Basin [Member] bbl Oil_well acre	Jun. 30, 2013 Reagan and Upton Counties Acreage Purchase [Member]	Jun. 30, 2013 Reagan and Upton Counties Acreage Purchase, Unevaluated [Member]	Jun. 30, 2013 Reagan and Upton Counties Acreage Purchase, Evaluated [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loss on Sale of Assets and Asset Impairment Charges	$ 1,707	$ 1,177	$ 0
Gas and Oil Area, Developed, Gross				2,468
Gas and Oil Area, Developed, Net				2,186
Business Combination, Consideration Transferred				11,000
Productive Oil Wells, Number of Wells, Gross				7
Proved Developed Reserves (Volume)				1,301,000
Business Combination, Consideration Transferred					$ 11,000	$ 2,000	$ 9,000