Supplementary Oil and Natural Gas Disclosures (Narrative) (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2014 USD ($) MBoe	Jun. 30, 2015 MBoe site	Jun. 30, 2015 site $ / Mcf	Jun. 30, 2015 MMBoe site	Jun. 30, 2015 site $ / MMBTU	Jun. 30, 2015 site $ / bbl	Jun. 30, 2015 site	Jun. 30, 2015 USD ($) site	Jun. 30, 2014 MMBoe	Dec. 31, 2013 USD ($) MBoe	Dec. 31, 2012 USD ($) MBoe
Reserve Quantities [Line Items]
Development costs | $	$ 242,217,000							$ 244,216,000		$ 303,245,000	$ 180,938,000
Proved Developed Reserves [Member]
Reserve Quantities [Line Items]
Increase (decrease) in proved reserve estimates				(18.4)
Percentage decrease in proved developed reserve estimates							30.00%
Proved developed reserve, MMBOE				43.7					62.1
Proved developed reserves downward revision				3.6
Divestiture				11.0
Production				9.3
Extensions and discoveries (Energy)				2.9
Proved undeveloped reserves transferred to developed				2.6
Proved Undeveloped Reserves [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserves transferred to developed				2.6
Increase (decrease) in proved undeveloped reserve estimates				(13.9)
Proved undeveloped reserve				13.5					27.4
Percentage decrease in proved undeveloped reserve estimate									51.00%
Proved undeveloped reserve, downward revision				11.2
Proved undeveloped reserves reclassified to proved developed, percentage							9.40%
Wells developed, gross | site		8	8	8	8	8	8	8
Wells developed, net | site		8	8	8	8	8	8	8
Development costs | $								$ 80,000,000
Proved Undeveloped Reserves [Member] | Effect of Reduced Oil and Gas Price [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserve, downward revision				1.8
Proved Undeveloped Reserves [Member] | No Longer Scheduled for Development Within Five Years [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserve, downward revision				2.8
Proved Undeveloped Reserves [Member] | New Data and Field Studies [Member]
Reserve Quantities [Line Items]
Proved undeveloped reserve, downward revision				6.6
Crude Oil (Mbbls) [Member] | West Texas Intermediate [Member]
Reserve Quantities [Line Items]
Commodity prices used in determining future cash flows before differentials | $ / bbl						68.17
Weighted average adjusted product price | $ / bbl						73.88
Unweighted average price for first day of month | $ / bbl						68.17
Commodity price differential between realized and first day of the month WTI | $ / bbl						5.71
Gas (Mmcf) [Member] | West Texas Intermediate [Member]
Reserve Quantities [Line Items]
Weighted average adjusted product price | $ / bbl						31.64
Gas (Mmcf) [Member] | Henry Hub [Member]
Reserve Quantities [Line Items]
Weighted average adjusted product price			3.11		3.39
Oil Equivalent (Mboe) [Member]
Reserve Quantities [Line Items]
Extensions and discoveries (Energy) | MBoe	1,310	3,239								10,729	8,095
3 Fields [Member] | Proved Undeveloped Reserves [Member] | New Data and Field Studies [Member]
Reserve Quantities [Line Items]
Percentage of downward revisions due to new data and field studies							75.00%
4 Fields [Member] | Proved Developed Reserves [Member] | New Data and Field Studies [Member]
Reserve Quantities [Line Items]
Percentage of downward revisions due to new data and field studies							82.00%