Supplemental Oil and Gas Information (Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 48,934.1	$ 50,150.5	$ 43,809.2
Future development costs	(6,882.7)	(7,161.5)	(6,786.9)
Future production costs	(18,724.3)	(16,808.1)	(14,684.7)
Future income taxes	(5,569.6)	(6,696.2)	(6,791.1)
Future net cash flows	17,757.5	19,484.7	15,546.5
10% annual discount for estimated timing of cash flows	(7,852.3)	(8,640.0)	(7,433.7)
Standardized measure of discounted future net cash flows	9,905.2	10,844.7	8,112.8	7,923.2
United States [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	20,767.4	20,638.6	15,547.5
Future development costs	(3,151.4)	(3,833.9)	(3,731.6)
Future production costs	(6,378.5)	(5,244.7)	(3,466.6)
Future income taxes	(2,930.1)	(3,368.3)	(2,527.6)
Future net cash flows	8,307.4	8,191.7	5,821.7
10% annual discount for estimated timing of cash flows	(3,729.1)	(4,020.2)	(2,862.1)
Standardized measure of discounted future net cash flows	4,578.3	4,171.5	2,959.6
Canada [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	16,257.0	16,112.9	15,511.6
Future development costs	(1,810.5)	(1,882.3)	(1,815.2)
Future production costs	(7,770.2)	(7,073.0)	(7,336.4)
Future income taxes	(1,389.6)	(1,472.8)	(1,714.9)
Future net cash flows	5,286.7	5,684.8	4,645.1
10% annual discount for estimated timing of cash flows	(2,595.3)	(2,999.1)	(2,876.5)
Standardized measure of discounted future net cash flows	2,691.4	2,685.7	1,768.6
Malaysia [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	11,909.7	13,399.0	10,354.9
Future development costs	(1,920.8)	(1,445.3)	(966.9)
Future production costs	(4,575.6)	(4,490.4)	(3,143.4)
Future income taxes	(1,249.9)	(1,855.1)	(1,675.9)
Future net cash flows	4,163.4	5,608.2	4,568.7
10% annual discount for estimated timing of cash flows	(1,527.9)	(1,620.7)	(1,322.9)
Standardized measure of discounted future net cash flows	2,635.5	3,987.5	3,245.8
United Kingdom [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows			2,395.2
Future development costs			(273.2)
Future production costs			(738.3)
Future income taxes			(872.7)
Future net cash flows			511.0
10% annual discount for estimated timing of cash flows			(372.2)
Standardized measure of discounted future net cash flows			$ 138.8

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Amount of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for costs to be incurred in developing proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for income tax, based on the future income less future expense derived from production and sale of proved oil and gas reserves.

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Amount, after future accumulated cash outflow for development and production costs and future income tax, of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and gas reserves.

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Amount of discount on future accumulated cash inflow, after deducting future accumulated cash outflow for development and production costs and future income tax, from sale of proved oil and gas reserves, using a discount rate of ten percent a year.

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Amount of discounted future accumulated cash inflow relating to proved oil and gas reserves.

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