Supplemental Information on Oil and Gas Operations (Unaudited) (Proved Developed and Undeveloped Reserves) (Details) MBbls in Thousands, Mcf in Millions		12 Months Ended
		Dec. 31, 2019 MMBoe MBbls Mcf	Dec. 31, 2018 MMBoe MBbls Mcf	Dec. 31, 2017 MMBoe MBbls Mcf	Dec. 31, 2016 MMBoe MBbls Mcf
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | MMBoe	[1]	823	787	660
Proved developed and undeveloped reserves, revisions due to prices | MMBoe	[1]	(28)	15	27
Proved developed and undeveloped reserves, revisions other than price | MMBoe	[1]	(31)	(53)	(14)
Proved developed and undeveloped reserves, extensions and discoveries | MMBoe	[1]	160	206	215
Proved developed and undeveloped reserves, purchase of reserves | MMBoe	[1]	6
Proved developed and undeveloped reserves, production | MMBoe	[1]	(119)	(108)	(95)
Proved developed and undeveloped reserves, sale of reserves | MMBoe	[1]	(54)	(24)	(6)
Proved developed and undeveloped reserves, ending balance | MMBoe	[1]	757	823	787
Proved developed reserves | MMBoe	[1]	589	600	585	545
Proved developed producing reserves | MMBoe	[1]	578	582	554	500
Proved undeveloped reserves | MMBoe	[1]	168	223	202	115
Conversion rate of gas reserves from barrels of oil to Boe		6
Oil [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		296	254	191
Proved developed and undeveloped reserves, revisions due to prices		(7)	12	12
Proved developed and undeveloped reserves, revisions other than price		(13)	(10)	6
Proved developed and undeveloped reserves, extensions and discoveries		76	93	90
Proved developed and undeveloped reserves, purchase of reserves		3
Proved developed and undeveloped reserves, production		(55)	(47)	(42)
Proved developed and undeveloped reserves, sale of reserves		(24)	(6)	(3)
Proved developed and undeveloped reserves, ending balance		276	296	254
Proved developed reserves		198	196	175	157
Proved developed producing reserves		191	188	163	141
Proved undeveloped reserves		78	100	79	34
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | Mcf		1,802	1,810	1,613
Proved developed and undeveloped reserves, revisions due to prices | Mcf		(86)	7	55
Proved developed and undeveloped reserves, revisions other than price | Mcf		(50)	(102)	(31)
Proved developed and undeveloped reserves, extensions and discoveries | Mcf		269	358	371
Proved developed and undeveloped reserves, purchase of reserves | Mcf		7
Proved developed and undeveloped reserves, production | Mcf		(219)	(206)	(189)
Proved developed and undeveloped reserves, sale of reserves | Mcf		(102)	(65)	(9)
Proved developed and undeveloped reserves, ending balance | Mcf		1,621	1,802	1,810
Proved developed reserves | Mcf		1,344	1,427	1,455	1,359
Proved developed producing reserves | Mcf		1,327	1,394	1,384	1,267
Proved undeveloped reserves | Mcf		277	375	355	254
Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		227	231	200
Proved developed and undeveloped reserves, revisions due to prices		(6)	2	5
Proved developed and undeveloped reserves, revisions other than price		(9)	(27)	(15)
Proved developed and undeveloped reserves, extensions and discoveries		39	54	63
Proved developed and undeveloped reserves, purchase of reserves		1
Proved developed and undeveloped reserves, production		(28)	(26)	(21)
Proved developed and undeveloped reserves, sale of reserves		(13)	(7)	(1)
Proved developed and undeveloped reserves, ending balance		211	227	231
Proved developed reserves		167	166	168	161
Proved developed producing reserves		165	162	160	148
Proved undeveloped reserves		44	61	63	39

[1] Gas reserves are converted to Boe at the rate of six Mcf per Bbl of oil, based upon the approximate relative energy content of gas and oil. NGL reserves are converted to Boe on a one-to-one basis with oil. The conversion rates are not necessarily indicative of the relationship of oil, natural gas and NGL prices.