Supplemental Oil and Gas Disclosures (SM Future Net CS Inflows) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves [Line Items]
Future cash inflows	$ 2,066,466	$ 1,159,503
Future production costs	(578,515)	(191,450)
Future development costs	(582,173)	(371,385)
Future income tax expense	(277,436)	(238,111)
Future net cash inflows (undiscounted)	628,342	358,557
Annual discount of 10% for estimated timing	129,364	93,685
Standarized measure of future net cash flows	498,978	264,872
[GB]
Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves [Line Items]
Future cash inflows	2,035,208	941,208
Future production costs	(568,414)	(144,106)
Future development costs	(581,277)	(306,628)
Future income tax expense	(277,436)	(238,111)
Future net cash inflows (undiscounted)	608,081	252,363
Annual discount of 10% for estimated timing	122,780	37,250
Standarized measure of future net cash flows	485,301	215,113
[DomesticCountryMember]
Discounted Future Net Cash Flows Relating To Proved Oil And Gas Reserves [Line Items]
Future cash inflows	31,258	218,295
Future production costs	(10,101)	(47,344)
Future development costs	(896)	(64,757)
Future income tax expense	0	0
Future net cash inflows (undiscounted)	20,261	106,194
Annual discount of 10% for estimated timing	6,584	56,435
Standarized measure of future net cash flows	$ 13,677	$ 49,759