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, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 21,992.1	$ 17,549.3	$ 27,439.0
Future development costs	(3,793.5)	(3,598.5)	(4,723.7)
Future production costs	(9,577.7)	(8,972.5)	(14,138.6)
Future income taxes	(1,287.0)	(273.5)	(1,060.7)
Future net cash flows	7,333.9	4,704.8	7,516.0
10% annual discount for estimated timing of cash flows	(3,411.0)	(2,124.8)	(3,656.9)
Standardized measure of discounted future net cash flows	3,922.9	2,580.0	3,859.1	$ 9,905.2
United States [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	12,885.8	9,477.9	12,373.9
Future development costs	(2,079.5)	(1,691.1)	(2,620.5)
Future production costs	(4,765.3)	(3,981.6)	(4,955.4)
Future income taxes	(893.7)	(118.9)	(339.7)
Future net cash flows	5,147.3	3,686.3	4,458.3
10% annual discount for estimated timing of cash flows	(2,698.2)	(1,799.5)	(2,430.0)
Standardized measure of discounted future net cash flows	2,449.1	1,886.8	2,028.3
Canada [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	4,714.3	3,752.7	8,922.0
Future development costs	(1,081.7)	(1,143.6)	(1,145.4)
Future production costs	(2,507.4)	(2,329.7)	(5,892.7)
Future income taxes	(161.1)	(81.3)	(504.8)
Future net cash flows	964.1	198.1	1,379.1
10% annual discount for estimated timing of cash flows	(394.6)	(95.0)	(666.8)
Standardized measure of discounted future net cash flows	569.5	103.1	712.3
Malaysia [Member]
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	4,392.0	4,318.7	6,143.1
Future development costs	(632.3)	(763.8)	(957.8)
Future production costs	(2,305.0)	(2,661.2)	(3,290.5)
Future income taxes	(232.2)	(73.3)	(216.2)
Future net cash flows	1,222.5	820.4	1,678.6
10% annual discount for estimated timing of cash flows	(318.2)	(230.3)	(560.1)
Standardized measure of discounted future net cash flows	$ 904.3	$ 590.1	$ 1,118.5