Supplemental Information On Oil And Natural Gas Producing Activities (Proved Reserves Narrative) (Details) MMBoe in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) MMBoe $ / Mcf $ / bbl	Dec. 31, 2014 USD ($) MMBoe $ / Mcf $ / bbl	Dec. 31, 2013 MMBoe
Reserve Quantities [Line Items]
Exploration and dry hole costs | $	$ 0.0	$ 0.0
Proved reserve estimates	11,642	16,627	15,209
Reserve revision	11,523	12,439
Increase (decrease) in proved reserve estimates	5,000	1,400
Oil [Member]
Reserve Quantities [Line Items]
Proved reserve estimates	3,159	1,662	2,072
Reserve revision	3,071	1,523
Weighted-average product price | $ / bbl	50.28	93.95
Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Proved reserve estimates	747	58	143
Reserve revision	747	58
Weighted-average product price | $ / bbl	19.90	35.11
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved reserve estimates	7,736	14,907	12,994
Reserve revision	7,705	10,858
Percentage of reserves	66.00%	90.00%
Weighted-average product price | $ / Mcf	2.58	4.09
Cherokee Basin [Member]
Reserve Quantities [Line Items]
Increase (decrease) in proved reserve estimates	4,000	2,200