Supplementary Oil and Natural Gas Disclosures (Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Natural Gas Reserves) (Details) - USD ($) $ in Thousands		Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Oil and Natural Gas Disclosures [Abstract]
Future cash inflows		$ 3,055,918	$ 6,704,859
Future production costs		(1,114,204)	(2,126,110)
Future development costs	[1],[2]	(697,112)	(1,161,289)
Future income taxes			(712,700)
Future net cash flows after income taxes		1,244,602	2,704,760
10% annual discount for estimated timing of cash flows		(418,836)	(740,167)
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Total		$ 825,766	$ 1,964,593	$ 1,649,078	$ 1,574,282

[1] As of June 30,As ofJune 30,20152014(In thousands)Future cash inflows$ 3,055,918$ 6,704,859Future production costs (1,114,204) (2,126,110)Future development costs (697,112) (1,161,289)Future income taxes - (712,700)Future net cash flows after income taxes 1,244,602 2,704,76010% annual discount for estimated timing of cash flows (418,836) (740,167)Standardized measure of discounted future net cash flows$ 825,766$ 1,964,593

[2] As of June 30,As ofJune 30,20152014(In thousands)Future cash inflows$ 3,055,918$ 6,704,859Future production costs (1,114,204) (2,126,110)Future development costs (697,112) (1,161,289)Future income taxes - (712,700)Future net cash flows after income taxes 1,244,602 2,704,76010% annual discount for estimated timing of cash flows (418,836) (740,167)Standardized measure of discounted future net cash flows$ 825,766$ 1,964,593