Supplemental Gas and Oil Information (Unaudited) Standardized Measure Of Future Net Cash Flows (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Abstract]
Future cash inflows	$ 15,325.3	$ 28,167.3	$ 24,805.7
Future production costs	(7,389.9)	(9,842.1)	(8,400.3)
Future development costs	(2,202.5)	(3,521.3)	(4,056.7)
Future income tax expenses	(1,169.3)	(4,304.0)	(3,284.6)
Future net cash flows	4,563.6	10,499.9	9,064.1
10% annual discount for estimated timing of net cash flows	(2,087.3)	(5,159.9)	(4,680.2)
Standardized measure of discounted future net cash flows	$ 2,476.3	$ 5,340.0	$ 4,383.9	$ 3,034.7

No definition available.

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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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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 31
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 30
-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 30
-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=34713648&loc=d3e63123-109448

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62500-109447

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 31
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447

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