Supplemental Oil and Gas Information (Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	[1]	$ 34,423.0	$ 21,992.1	$ 17,549.3
Future development costs	[1]	(5,159.2)	(3,793.5)	(3,598.5)
Future production costs	[1]	(12,785.9)	(9,577.7)	(8,972.5)
Future income taxes	[1]	(2,924.9)	(1,287.0)	(273.5)
Future net cash flows	[1]	13,553.0	7,333.9	4,704.8
10% annual discount for estimated timing of cash flows	[1]	(5,273.1)	(3,411.0)	(2,124.8)
Standardized measure of discounted future net cash flows	[1]	8,279.9	3,922.9	2,580.0	$ 3,859.1
United States [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	[1]	23,473.9	12,885.8	9,477.9
Future development costs	[1]	(3,279.1)	(2,079.5)	(1,691.1)
Future production costs	[1]	(7,279.5)	(4,765.3)	(3,981.6)
Future income taxes	[1]	(2,216.5)	(893.7)	(118.9)
Future net cash flows	[1]	10,698.8	5,147.3	3,686.3
10% annual discount for estimated timing of cash flows	[1]	(4,295.4)	(2,698.2)	(1,799.5)
Standardized measure of discounted future net cash flows	[1]	6,403.4	2,449.1	1,886.8
Canada [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	[1]	5,437.5	4,714.3	3,752.7
Future development costs	[1]	(1,362.7)	(1,081.7)	(1,143.6)
Future production costs	[1]	(2,693.0)	(2,507.4)	(2,329.7)
Future income taxes	[1]	(236.4)	(161.1)	(81.3)
Future net cash flows	[1]	1,145.4	964.1	198.1
10% annual discount for estimated timing of cash flows	[1]	(531.4)	(394.6)	(95.0)
Standardized measure of discounted future net cash flows	[1]	614.0	569.5	103.1
Malaysia [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	[1]	5,511.6	4,392.0	4,318.7
Future development costs	[1]	(517.4)	(632.3)	(763.8)
Future production costs	[1]	(2,813.4)	(2,305.0)	(2,661.2)
Future income taxes	[1]	(472.0)	(232.2)	(73.3)
Future net cash flows	[1]	1,708.8	1,222.5	820.4
10% annual discount for estimated timing of cash flows	[1]	(446.3)	(318.2)	(230.3)
Standardized measure of discounted future net cash flows	[1]	$ 1,262.5	$ 904.3	$ 590.1

[1] 2018 includes noncontrolling interest in MP GOM.