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, 2015 MMBbls ft³	Dec. 31, 2014 MMBbls ft³	Dec. 31, 2013 MMBbls ft³	Dec. 31, 2012 MMBbls ft³
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	441.8	471.2	414.8
Revisions of previous estimates	5.3	(9.3)	27.4
Improved recovery	2.4	7.5	27.4
Extensions and discoveries	63.8	42.6	69.6
Purchases of properties		6.1
Sales of properties	(11.0)	(24.3)	(20.4)
Production	(46.1)	(52.0)	(47.6)
Proved developed and undeveloped reserves, Ending Balance	456.2	441.8	471.2
Proved developed reserves	326.6	324.1	289.9	267.7
Proved undeveloped reserves	129.6	117.7	181.3	147.1
Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	336.2	354.2	295.7
Revisions of previous estimates	(8.2)	(2.3)	24.8
Improved recovery	2.4	7.5	27.4
Extensions and discoveries	63.8	42.6	69.6
Purchases of properties		6.1
Sales of properties	(11.0)	(24.3)	(20.4)
Production	(41.8)	(47.6)	(42.9)
Proved developed and undeveloped reserves, Ending Balance	341.4	336.2	354.2
Proved developed reserves	211.8	218.5	172.9	148.6
Proved undeveloped reserves	129.6	117.7	181.3	147.1
Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	30.6	24.4
Revisions of previous estimates	2.0	5.1	15.7
Extensions and discoveries	7.6	4.7	10.0
Sales of properties	(0.1)	(0.2)
Production	(3.7)	(3.4)	(1.3)
Proved developed and undeveloped reserves, Ending Balance	36.4	30.6	24.4
Proved developed reserves	21.6	17.5	14.2
Proved undeveloped reserves	14.8	13.1	10.2
Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance | ft³	1,704.7	1,153.6	1,137.0
Revisions of previous estimates | ft³	53.5	167.2	33.7
Improved recovery | ft³	1.8	7.0	3.2
Extensions and discoveries | ft³	162.9	696.8	153.4
Purchases of properties | ft³		5.5
Sales of properties | ft³	(78.0)	(162.6)	(19.0)
Production | ft³	(156.1)	(162.8)	(154.7)
Proved developed and undeveloped reserves, Ending Balance | ft³	1,688.8	1,704.7	1,153.6
Proved developed reserves | ft³	783.5	812.1	786.2	706.0
Proved undeveloped reserves | ft³	905.3	892.6	367.4	431.0
United States [Member] | Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	204.9	191.5	142.6
Revisions of previous estimates	(7.6)	(3.2)	13.1
Extensions and discoveries	63.8	32.7	52.4
Purchases of properties		6.1
Sales of properties		(0.3)
Production	(22.2)	(21.9)	(16.6)
Proved developed and undeveloped reserves, Ending Balance	238.9	204.9	191.5
Proved developed reserves	125.9	106.2	75.8	48.0
Proved undeveloped reserves	113.0	98.7	115.7	94.6
United States [Member] | Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	29.1	23.2
Revisions of previous estimates	2.2	5.0	15.6
Extensions and discoveries	7.6	4.0	8.7
Production	(3.5)	(3.1)	(1.1)
Proved developed and undeveloped reserves, Ending Balance	35.4	29.1	23.2
Proved developed reserves	20.7	16.5	13.1
Proved undeveloped reserves	14.7	12.6	10.1
United States [Member] | Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance | ft³	226.3	185.0	209.7
Revisions of previous estimates | ft³	(5.2)	47.7	(38.6)
Extensions and discoveries | ft³	43.2	24.1	33.3
Purchases of properties | ft³		5.5
Sales of properties | ft³		(3.7)
Production | ft³	(31.9)	(32.3)	(19.4)
Proved developed and undeveloped reserves, Ending Balance | ft³	232.4	226.3	185.0
Proved developed reserves | ft³	148.3	145.6	112.6	78.8
Proved undeveloped reserves | ft³	84.1	80.7	72.4	130.9
Canada [Member] | Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	37.4	38.7	36.8
Revisions of previous estimates	(4.8)	2.7	8.4
Extensions and discoveries		2.4	0.2
Sales of properties		(0.5)
Production	(4.7)	(5.9)	(6.7)
Proved developed and undeveloped reserves, Ending Balance	27.9	37.4	38.7
Proved developed reserves	23.8	32.4	31.6	29.5
Proved undeveloped reserves	4.1	5.0	7.1	7.3
Canada [Member] | Synthetic Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	105.6	117.0	119.1
Revisions of previous estimates	13.5	(7.0)	2.6
Production	(4.3)	(4.4)	(4.7)
Proved developed and undeveloped reserves, Ending Balance	114.8	105.6	117.0
Proved developed reserves	114.8	105.6	117.0	119.1
Canada [Member] | Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	0.7	0.1
Revisions of previous estimates	(0.3)
Extensions and discoveries		0.6	0.1
Proved developed and undeveloped reserves, Ending Balance	0.4	0.7	0.1
Proved developed reserves	0.3	0.2
Proved undeveloped reserves	0.1	0.5	0.1
Canada [Member] | Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance | ft³	842.8	562.8	550.4
Revisions of previous estimates | ft³	18.9	105.6	34.0
Extensions and discoveries | ft³	119.7	231.5	42.5
Production | ft³	(71.8)	(57.1)	(64.1)
Proved developed and undeveloped reserves, Ending Balance | ft³	909.6	842.8	562.8
Proved developed reserves | ft³	453.5	467.4	384.0	415.8
Proved undeveloped reserves | ft³	456.1	375.4	178.8	134.6
Malaysia [Member] | Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	93.9	124.0	95.7
Revisions of previous estimates	4.2	(1.8)	3.3
Improved recovery	2.4	7.5	27.4
Extensions and discoveries		7.5	17.0
Sales of properties	(11.0)	(23.5)
Production	(14.9)	(19.8)	(19.4)
Proved developed and undeveloped reserves, Ending Balance	74.6	93.9	124.0
Proved developed reserves	62.1	79.9	65.5	67.0
Proved undeveloped reserves	12.5	14.0	58.5	28.7
Malaysia [Member] | Natural Gas Liquids Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance	0.8	1.1
Revisions of previous estimates	0.1	0.1	0.1
Extensions and discoveries		0.1	1.2
Sales of properties	(0.1)	(0.2)
Production	(0.2)	(0.3)	(0.2)
Proved developed and undeveloped reserves, Ending Balance	0.6	0.8	1.1
Proved developed reserves	0.6	0.8	1.1
Malaysia [Member] | Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance | ft³	635.6	405.8	357.6
Revisions of previous estimates | ft³	39.8	13.9	38.3
Improved recovery | ft³	1.8	7.0	3.2
Extensions and discoveries | ft³		441.2	77.6
Sales of properties | ft³	(78.0)	(158.9)
Production | ft³	(52.4)	(73.4)	(70.9)
Proved developed and undeveloped reserves, Ending Balance | ft³	546.8	635.6	405.8
Proved developed reserves | ft³	181.7	199.1	289.6	197.3
Proved undeveloped reserves | ft³	365.1	436.5	116.2	160.3
United Kingdom [Member] | Crude Oil [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance			20.6
Sales of properties			(20.4)
Production			(0.2)
Proved developed reserves				4.1
Proved undeveloped reserves				16.5
United Kingdom [Member] | Natural Gas Reserves [Member]
Proved developed and undeveloped oil reserves:
Proved developed and undeveloped reserves, Beginning Balance | ft³			19.3
Sales of properties | ft³			(19.0)
Production | ft³			(0.3)
Proved developed reserves | ft³				14.1
Proved undeveloped reserves | ft³				5.2