Supplemental Oil and Gas Information (Summary of Proved Reserves Based on Average Prices) (Details) MMBbls in Millions, ft³ in Billions		12 Months Ended
		Dec. 31, 2017 MMcf ft³ MMBbls	Dec. 31, 2017 ft³ MMBbls	Dec. 31, 2017 ft³ MMBbls	Dec. 31, 2016 ft³ MMBbls	Dec. 31, 2015 ft³ MMBbls	Dec. 31, 2014 ft³ MMBbls
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				329.0	456.2	441.8
Revisions of previous estimates				(6.0)	(5.8)	5.3
Improved recovery				2.0		2.4
Extensions and discoveries				31.6	11.0	63.8
Purchases of properties				4.7	26.3
Sales of properties					(121.0)	(11.0)
Production				(33.2)	(37.7)	(46.1)
Proved developed and undeveloped reserves, Ending Balance				328.1	329.0	456.2
Proved developed reserves		185.5	185.5	185.5	184.9	326.6	324.1
Proved undeveloped reserves		142.6	142.6	142.6	144.1	129.6	117.7
Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				329.0	341.4	336.2
Revisions of previous estimates				(6.0)	(5.8)	(8.2)
Improved recovery				2.0		2.4
Extensions and discoveries				31.6	11.0	63.8
Purchases of properties				4.7	26.3
Sales of properties					(7.8)	(11.0)
Production				(33.2)	(36.1)	(41.8)
Proved developed and undeveloped reserves, Ending Balance				328.1	329.0	341.4
Proved developed reserves		185.5	185.5	185.5	184.9	211.8	218.5
Proved undeveloped reserves		142.6	142.6	142.6	144.1	129.6	117.7
Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				42.5	36.4	30.6
Revisions of previous estimates				1.3	1.6	2.0
Extensions and discoveries				7.8	2.9	7.6
Purchases of properties				0.5	5.1
Sales of properties						(0.1)
Production				(3.2)	(3.5)	(3.7)
Proved developed and undeveloped reserves, Ending Balance				48.9	42.5	36.4
Proved developed reserves		24.6	24.6	24.6	22.2	21.6	17.5
Proved undeveloped reserves		24.3	24.3	24.3	20.3	14.8	13.1
Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance | ft³			1,878.0		1,688.8	1,704.7
Revisions of previous estimates | ft³			(5.4)		43.3	53.5
Improved recovery | ft³						1.8
Extensions and discoveries | ft³			190.6		164.2	162.9
Purchases of properties | ft³			4.0		122.3
Sales of properties | ft³					(2.2)	(78.0)
Production | ft³			(140.1)		(138.4)	(156.1)
Proved developed and undeveloped reserves, Ending Balance | ft³			1,927.1		1,878.0	1,688.8
Proved developed reserves | ft³		819.3	819.3	819.3	818.1	783.5	812.1
Proved undeveloped reserves | ft³		1,107.8	1,107.8	1,107.8	1,059.9	905.3	892.6
United States [Member] | Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				214.4	238.9	204.9
Revisions of previous estimates				(4.7)	(10.9)	(7.6)
Extensions and discoveries				27.2	8.6	63.8
Purchases of properties				4.7
Sales of properties					(4.5)
Production				(16.9)	(17.7)	(22.2)
Proved developed and undeveloped reserves, Ending Balance				224.7	214.4	238.9
Proved developed reserves		126.3	126.3	126.3	113.9	125.9	106.2
Proved undeveloped reserves		98.4	98.4	98.4	100.5	113.0	98.7
United States [Member] | Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				36.4	35.4	29.1
Revisions of previous estimates				2.0	1.2	2.2
Extensions and discoveries				7.0	2.8	7.6
Purchases of properties				0.5
Production				(2.9)	(3.0)	(3.5)
Proved developed and undeveloped reserves, Ending Balance				43.0	36.4	35.4
Proved developed reserves		23.3	23.3	23.3	20.8	20.7	16.5
Proved undeveloped reserves		19.7	19.7	19.7	15.6	14.7	12.6
United States [Member] | Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance | ft³			219.4		232.4	226.3
Revisions of previous estimates | ft³			(16.0)		0.1	(5.2)
Extensions and discoveries | ft³			32.2		6.4	43.2
Purchases of properties | ft³			4.0
Sales of properties | ft³					(0.1)
Production | ft³			(16.3)		(19.4)	(31.9)
Proved developed and undeveloped reserves, Ending Balance | ft³			223.3		219.4	232.4
Proved developed reserves | ft³		127.7	127.7	127.7	138.7	148.3	145.6
Proved undeveloped reserves | ft³		95.6	95.6	95.6	80.7	84.1	80.7
Canada [Member] | Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				48.9	27.9	37.4
Revisions of previous estimates				2.3	2.5	(4.8)
Extensions and discoveries				4.4
Purchases of properties					26.3
Sales of properties					(3.3)
Production				(4.1)	(4.5)	(4.7)
Proved developed and undeveloped reserves, Ending Balance				51.5	48.9	27.9
Proved developed reserves		21.9	21.9	21.9	19.2	23.8	32.4
Proved undeveloped reserves		29.6	29.6	29.6	29.7	4.1	5.0
Canada [Member] | Synthetic Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance	[1]			0.0	114.8	105.6
Revisions of previous estimates	[1]					13.5
Sales of properties	[1]				(113.2)
Production	[1]				(1.6)	(4.3)
Proved developed and undeveloped reserves, Ending Balance	[1]				0.0	114.8
Proved developed reserves	[1]					114.8	105.6
Canada [Member] | Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				5.6	0.4	0.7
Revisions of previous estimates				(0.6)	0.2	(0.3)
Extensions and discoveries				0.8	0.1
Purchases of properties					5.1
Production				(0.2)	(0.2)
Proved developed and undeveloped reserves, Ending Balance				5.6	5.6	0.4
Proved developed reserves		1.0	1.0	1.0	0.9	0.3	0.2
Proved undeveloped reserves		4.6	4.6	4.6	4.7	0.1	0.5
Canada [Member] | Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance | ft³			1,118.9		909.6	842.8
Revisions of previous estimates | ft³			19.4		45.3	18.9
Extensions and discoveries | ft³			156.7		120.2	119.7
Purchases of properties | ft³					122.3
Sales of properties | ft³					(2.1)
Production | ft³			(82.6)		(76.4)	(71.8)
Proved developed and undeveloped reserves, Ending Balance | ft³			1,212.4		1,118.9	909.6
Proved developed reserves | ft³		547.0	547.0	547.0	498.9	453.5	467.4
Proved undeveloped reserves | ft³		665.5	665.5	665.5	620.0	456.1	375.4
Malaysia [Member] | Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				65.7	74.6	93.9
Revisions of previous estimates				(3.6)	2.6	4.2
Improved recovery				2.0		2.4
Extensions and discoveries					2.4
Sales of properties						(11.0)
Production				(12.2)	(13.9)	(14.9)
Proved developed and undeveloped reserves, Ending Balance				51.9	65.7	74.6
Proved developed reserves		37.3	37.3	37.3	51.8	62.1	79.9
Proved undeveloped reserves		14.6	14.6	14.6	13.9	12.5	14.0
Malaysia [Member] | Oil Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Ending Balance				52.2
Malaysia [Member] | Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance				0.5	0.6	0.8
Revisions of previous estimates				(0.1)	0.2	0.1
Sales of properties						(0.1)
Production				(0.1)	(0.3)	(0.2)
Proved developed and undeveloped reserves, Ending Balance				0.3	0.5	0.6
Proved developed reserves		0.3	0.3	0.3	0.5	0.6	0.8
Malaysia [Member] | Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped crude oil/synthetic oil reserves, Beginning Balance | ft³			539.7		546.8	635.6
Revisions of previous estimates | ft³			(8.8)		(2.1)	39.8
Improved recovery | ft³						1.8
Extensions and discoveries | ft³			1.7		37.6
Sales of properties | ft³						(78.0)
Production | ft³			(41.2)		(42.6)	(52.4)
Proved developed and undeveloped reserves, Ending Balance		491,300	491.4		539.7	546.8
Proved developed reserves | ft³		144.6	144.6	144.6	180.5	181.7	199.1
Proved undeveloped reserves | ft³		346.7	346.7	346.7	359.2	365.1	436.5

[1] All synthetic oil operations were sold in June 2016.