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Supplementary Oil and Natural Gas Disclosures (Unaudited) (Schedule of Changes in Standardized Measure of Discounted Future Net Cash Flows Applicable to Proved Oil and Natural Gas Reserves) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Supplementary Oil and Natural Gas Disclosures (Unaudited) [Abstract]    
Beginning of the period $ 1,574,282 $ 876,169
Sales and transfers of oil and natural gas produced, net of productuin costs (513,906) (317,059)
Net changes in prices and prooduction costs 96,751 (244,824)
Purchase of mineral in place 26,708 797,085
Sales of minerals in place (64,539)  
Extensions, discoveries and improved recoveries, net of future production costs 363,493 452,258
Revision of quantity estimates 84,490 55,133
Previously estimated development costs incurred during the period 33,920 39,321
Changes in estimated future development costs 18,229 746
Changes in production rates (timing) and other (113,022) (14,157)
Accretion of discount 197,927 110,070
Net change in income taxes (55,255) (180,460)
Net increase (decrease) 74,796 698,113
End of period $ 1,649,078 $ 1,574,282