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, 2020 MMBoe MBbls Mcf	Dec. 31, 2019 MMBoe MBbls Mcf	Dec. 31, 2018 MMBoe MBbls Mcf	Dec. 31, 2017 MMBoe MBbls Mcf
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | MMBoe	[1]	757	823	787
Proved developed and undeveloped reserves, revisions due to prices | MMBoe	[1]	(78)	(28)	15
Proved developed and undeveloped reserves, revisions other than price | MMBoe	[1]	55	(31)	(53)
Proved developed and undeveloped reserves, extensions and discoveries | MMBoe	[1]	135	160	206
Proved developed and undeveloped reserves, purchase of reserves | MMBoe	[1]	7	6
Proved developed and undeveloped reserves, production | MMBoe	[1]	(122)	(119)	(108)
Proved developed and undeveloped reserves, sale of reserves | MMBoe	[1]	(2)	(54)	(24)
Proved developed and undeveloped reserves, ending balance | MMBoe	[1]	752	757	823
Proved developed reserves | MMBoe	[1]	574	589	600	585
Proved developed producing reserves | MMBoe	[1]	564	578	582	554
Proved undeveloped reserves | MMBoe	[1]	178	168	223	202
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		276	296	254
Proved developed and undeveloped reserves, revisions due to prices		(26)	(7)	12
Proved developed and undeveloped reserves, revisions other than price		18	(13)	(10)
Proved developed and undeveloped reserves, extensions and discoveries		71	76	93
Proved developed and undeveloped reserves, purchase of reserves		1	3
Proved developed and undeveloped reserves, production		(57)	(55)	(47)
Proved developed and undeveloped reserves, sale of reserves		(1)	(24)	(6)
Proved developed and undeveloped reserves, ending balance		282	276	296
Proved developed reserves		194	198	196	175
Proved developed producing reserves		190	191	188	163
Proved undeveloped reserves		88	78	100	79
Natural Gas [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance | Mcf		1,621	1,802	1,810
Proved developed and undeveloped reserves, revisions due to prices | Mcf		(209)	(86)	7
Proved developed and undeveloped reserves, revisions other than price | Mcf		119	(50)	(102)
Proved developed and undeveloped reserves, extensions and discoveries | Mcf		188	269	358
Proved developed and undeveloped reserves, purchase of reserves | Mcf		19	7
Proved developed and undeveloped reserves, production | Mcf		(221)	(219)	(206)
Proved developed and undeveloped reserves, sale of reserves | Mcf		(5)	(102)	(65)
Proved developed and undeveloped reserves, ending balance | Mcf		1,512	1,621	1,802
Proved developed reserves | Mcf		1,244	1,344	1,427	1,455
Proved developed producing reserves | Mcf		1,223	1,327	1,394	1,384
Proved undeveloped reserves | Mcf		268	277	375	355
Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Proved developed and undeveloped reserves, beginning balance		211	227	231
Proved developed and undeveloped reserves, revisions due to prices		(17)	(6)	2
Proved developed and undeveloped reserves, revisions other than price		17	(9)	(27)
Proved developed and undeveloped reserves, extensions and discoveries		33	39	54
Proved developed and undeveloped reserves, purchase of reserves		3	1
Proved developed and undeveloped reserves, production		(28)	(28)	(26)
Proved developed and undeveloped reserves, sale of reserves		(1)	(13)	(7)
Proved developed and undeveloped reserves, ending balance		218	211	227
Proved developed reserves		173	167	166	168
Proved developed producing reserves		171	165	162	160
Proved undeveloped reserves		45	44	61	63

[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.