Supplemental Oil and Gas Disclosures (Narrative) (Details) - Oil Equivalents [Member] Boe in Thousands	12 Months Ended
	Dec. 31, 2017 Boe location	Dec. 31, 2016 Boe location	Dec. 31, 2015 Boe location
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy	4,747	3,424	(11,324)
Extensions and discoveries	19,768	1,537	14,332
Purchase of minerals in place	189
Sales of minerals in place	1,312	1,232	43
Proved Developed and Undeveloped Reserves, Production, Energy	2,698	2,262	2,181
Powder River Basin [Member]
Reserve Quantities [Line Items]
Sales of minerals in place	1,312
San Patricio County, Texas [Member]
Reserve Quantities [Line Items]
Sales of minerals in place		1,232
Continued improvement in Well Production Results [Member] | McKenzie County, ND [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy		5,005
Change in Projections [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy	621
Increase In Projections [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy	1,951
Increase in Economic LIfe Expectations [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy	2,698
Addition of New Producing Wells [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Extensions and discoveries	1,229	449
Addition of New Producing Wells [Member] | Atascosa County, TX [Member]
Reserve Quantities [Line Items]
Extensions and discoveries	240
Conversion of Probable Undeveloped Locations to Proved Producing Reserves [Member]
Reserve Quantities [Line Items]
Extensions and discoveries	2,028
Proved Undeveloped Locations Added | location	3
Addition of Proved Undeveloped Locations [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Extensions and discoveries	11,928	805	6,500
Proved Undeveloped Locations Added | location	27	5	6
Addition of Proved Undeveloped Locations [Member] | McKenzie County, ND [Member]
Reserve Quantities [Line Items]
Extensions and discoveries		18	6,500
Proved Undeveloped Locations Added | location		8	28
Addition of Proved Undeveloped Locations [Member] | North Dakota - Three Forks [Member]
Reserve Quantities [Line Items]
Extensions and discoveries			4,900
Proved Undeveloped Locations Added | location			20
Addition of Proved Undeveloped Locations [Member] | North Dakota - Bakken and Three Forks [Member]
Reserve Quantities [Line Items]
Extensions and discoveries			1,600
Proved Undeveloped Locations Added | location			8
Lack of Economic Viability at Lower Commodity Pricing [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy			(7,900)
Proved Undeveloped Locations Added | location			42
Proved Developed and Undeveloped Locations Added | location			48
Lack of Economic Viability at Lower Commodity Pricing [Member] | South Texas Eagle Ford Locations [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy			(7,717)
Conversion of Probable Undeveloped Locations to Proved Undeveloped Reserves [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Extensions and discoveries	4,343
Proved Undeveloped Locations Added | location	10
Conversion of Probable Undeveloped Locations to Proved Undeveloped Reserves [Member] | North Dakota - Bakken [Member]
Reserve Quantities [Line Items]
Extensions and discoveries			1,300
Proved Undeveloped Locations Added | location			21
Improved Performance [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy		1,360
Under-performance of Hedgehog State 16-2H [Member] | Porcupine Field, Campbell County, WY [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy		(670)
Shortened Economic Life Expectations [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy		(2,271)
Shortened Economic Life Expectations [Member] | South Texas Eagle Ford Locations [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy			(614)
Proved Developed and Undeveloped Locations Added | location			38
Shortened Economic Life Expectations [Member] | North Dakota - Bakken and Three Forks [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy			(1,800)
Increased Production [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy			600
Development of New Austin Chalk Producer [Member] | Atascosa County, TX [Member]
Reserve Quantities [Line Items]
Extensions and discoveries		265
Decreased Well Production [Member] | North Dakota - Bakken and Three Forks [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy			(1,600)
Change in Intent to Develop [Member] | Ward County, TX [Member]
Reserve Quantities [Line Items]
Proved Developed and Undeveloped Reserves, Production, Energy	523
Proved Undeveloped Locations Added | location	7