Supplementary Oil And Gas Information (Net Proved Reserves) (Details) MMcf in Thousands	12 Months Ended
Dec. 31, 2017 MMBbls MMcf	Dec. 31, 2016 MMBbls MMcf	Dec. 31, 2015 MMBbls MMcf	Dec. 31, 2017 MMBbls MMcf	Dec. 31, 2016 MMBbls MMcf	Dec. 31, 2015 MMBbls MMcf
Reserve Quantities [Line Items]
Beginning of year	789.7	799.4	1,277.4
Revisions of previous estimates	(43.6)	(64.7)	(321.1)
Extensions and discoveries	303.1	275.7	191.7
Purchase of reserves in place	1.5	14.9
Sale of reserves in place	(141.6)	(106.5)	(200.6)
Production	(114.3)	(129.1)	(148.0)
End of year	794.9	789.7	799.4
Developed				407.8	448.8	536.6
Undeveloped				387.1	341.0	262.8
Total	789.7	799.4	1,277.4	794.9	789.7	799.4
Oil [Member]
Reserve Quantities [Line Items]
Beginning of year	155.6	164.3	205.0
Revisions of previous estimates	(15.8)	(15.9)	(74.6)
Extensions and discoveries	85.1	52.2	68.4
Purchase of reserves in place	0.8	9.6
Sale of reserves in place	(5.4)	(27.6)	(2.8)
Production	(27.8)	(27.0)	(31.8)
End of year	192.5	155.6	164.3
Developed				104.9	82.5	96.6
Undeveloped				87.7	73.1	67.7
Total	155.6	164.3	205.0	192.5	155.6	164.3
Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Beginning of year	150.4	124.5	156.7
Revisions of previous estimates	(18.1)	(8.0)	(55.9)
Extensions and discoveries	72.9	75.8	44.7
Purchase of reserves in place	0.4	2.6
Sale of reserves in place	(3.8)	(26.8)	(4.0)
Production	(19.3)	(17.7)	(16.9)
End of year	182.5	150.4	124.5
Developed				82.1	57.4	69.5
Undeveloped				100.3	93.0	55.0
Total	150.4	124.5	156.7	182.5	150.4	124.5
Natural Gas [Member]
Reserve Quantities [Line Items]
Beginning of year \| MMcf	2,902	3,064	5,494
Revisions of previous estimates \| MMcf	(58)	(244)	(1,144)
Extensions and discoveries \| MMcf	871	887	472
Purchase of reserves in place \| MMcf	2	16
Sale of reserves in place \| MMcf	(795)	(313)	(1,163)
Production \| MMcf	(403)	(506)	(596)
End of year \| MMcf	2,519	2,902	3,064
Developed \| MMcf				1,325	1,853	2,223
Undeveloped \| MMcf				1,195	1,049	841
Total \| MMcf	2,902	3,064	5,494	2,519	2,902	3,064
Proved Developed [Member]
Reserve Quantities [Line Items]
Sale of reserves in place	(141.6)	(65.4)	(137.4)
Proved Undeveloped [Member]
Reserve Quantities [Line Items]
Sale of reserves in place		(41.2)	(63.2)
Canada [Member] \| Oil [Member]
Reserve Quantities [Line Items]
Beginning of year		6.4	10.9
Revisions of previous estimates	0.2	(0.3)	(0.9)
Extensions and discoveries	0.2
Sale of reserves in place		(5.4)	(1.6)
Production	(0.2)	(0.7)	(2.0)
End of year	0.2		6.4
Developed				0.2		5.0
Undeveloped						1.3
Total	0.2	6.4	10.9	0.2		6.4
Canada [Member] \| Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Beginning of year	94.0	62.8	66.6
Revisions of previous estimates	(14.6)	(6.4)	(14.8)
Extensions and discoveries	46.4	58.1	19.8
Sale of reserves in place	(0.2)	(11.3)	(0.4)
Production	(10.6)	(9.2)	(8.3)
End of year	115.0	94.0	62.8
Developed				40.5	25.6	31.8
Undeveloped				74.5	68.4	31.0
Total	94.0	62.8	66.6	115.0	94.0	62.8
Canada [Member] \| Natural Gas [Member]
Reserve Quantities [Line Items]
Beginning of year \| MMcf	1,810	1,952	3,229
Revisions of previous estimates \| MMcf	(31)	(422)	(801)
Extensions and discoveries \| MMcf	727	796	313
Sale of reserves in place \| MMcf	(65)	(163)	(434)
Production \| MMcf	(306)	(354)	(354)
End of year \| MMcf	2,135	1,810	1,952
Developed \| MMcf				1,082	903	1,295
Undeveloped \| MMcf				1,053	907	657
Total \| MMcf	1,810	1,952	3,229	2,135	1,810	1,952
United States [Member] \| Oil [Member]
Reserve Quantities [Line Items]
Beginning of year	155.6	157.9	194.1
Revisions of previous estimates	(16.0)	(15.6)	(73.6)
Extensions and discoveries	84.9	52.2	68.4
Purchase of reserves in place	0.8	9.6
Sale of reserves in place	(5.4)	(22.2)	(1.2)
Production	(27.7)	(26.2)	(29.7)
End of year	192.3	155.6	157.9
Developed				104.7	82.5	91.6
Undeveloped				87.7	73.1	66.3
Total	155.6	157.9	194.1	192.3	155.6	157.9
United States [Member] \| Natural Gas Liquids [Member]
Reserve Quantities [Line Items]
Beginning of year	56.4	61.7	90.2
Revisions of previous estimates	(3.6)	(1.6)	(41.1)
Extensions and discoveries	26.5	17.7	24.9
Purchase of reserves in place	0.4	2.6
Sale of reserves in place	(3.6)	(15.5)	(3.6)
Production	(8.7)	(8.5)	(8.6)
End of year	67.5	56.4	61.7
Developed				41.6	31.8	37.8
Undeveloped				25.8	24.6	24.0
Total	56.4	61.7	90.2	67.5	56.4	61.7
United States [Member] \| Natural Gas [Member]
Reserve Quantities [Line Items]
Beginning of year \| MMcf	1,093	1,112	2,265
Revisions of previous estimates \| MMcf	(27)	177	(342)
Extensions and discoveries \| MMcf	144	91	159
Purchase of reserves in place \| MMcf	2	16
Sale of reserves in place \| MMcf	(729)	(150)	(728)
Production \| MMcf	(97)	(153)	(241)
End of year \| MMcf	384	1,093	1,112
Developed \| MMcf				243	951	928
Undeveloped \| MMcf				141	142	184
Total \| MMcf	1,093	1,112	2,265	384	1,093	1,112

Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

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The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

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