Supplemental Oil and Gas Disclosures (SM Future Net CS Inflows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves [Line Items]
Future cash inflows	$ 828,812	$ 1,896,989
Future production costs	(297,755)	(583,549)
Future development costs	(357,473)	(594,563)
Future income tax expense	159,266	(133,495)
Future net cash inflows (undiscounted)	332,850	585,382
Annual discount of 10% for estimated timing	(190,885)	49,700
Standarized measure of future net cash flows	523,735	535,682
United Kingdom [Member]
Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves [Line Items]
Future cash inflows	778,119	1,862,137
Future production costs	(282,945)	(571,347)
Future development costs	(357,373)	(593,958)
Future income tax expense	159,266	(133,495)
Future net cash inflows (undiscounted)	297,067	563,337
Annual discount of 10% for estimated timing	(201,859)	43,152
Standarized measure of future net cash flows	498,926	520,185
United States [Member]
Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves [Line Items]
Future cash inflows	50,693	34,852
Future production costs	(14,810)	(12,202)
Future development costs	(100)	(605)
Future net cash inflows (undiscounted)	35,783	22,045
Annual discount of 10% for estimated timing	10,974	6,548
Standarized measure of future net cash flows	$ 24,809	$ 15,497