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Risk Management Activities (Schedule of Derivative Instruments in Statement of Financial Position, Fair Value) (Details) - USD ($)
$ in Millions

Mar. 31, 2017

Dec. 31, 2016

Risk Management Assets Current

Derivatives, Fair Value [Line Items]

Derivative Asset

^[1]

$ 20.0

$ 24.4

Risk Management Assets Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Asset

^[2]

18.9

24.6

Risk Management Liabilities Current

Derivatives, Fair Value [Line Items]

Derivative Liability

^[3]

12.4

16.8

Risk Management Liabilities Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Liability

52.9

44.5

Interest Rate Risk | Risk Management Assets Current

Derivatives, Fair Value [Line Items]

Derivative Asset

^[1]

16.9

17.0

Interest Rate Risk | Risk Management Assets Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Asset

^[2]

17.0

17.1

Interest Rate Risk | Risk Management Liabilities Current

Derivatives, Fair Value [Line Items]

Derivative Liability

^[3]

11.5

15.3

Interest Rate Risk | Risk Management Liabilities Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Liability

20.6

24.5

Commodity Price Risk Programs | Risk Management Assets Current

Derivatives, Fair Value [Line Items]

Derivative Asset

^[1]

3.1

7.4

Commodity Price Risk Programs | Risk Management Assets Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Asset

^[2]

1.9

7.5

Commodity Price Risk Programs | Risk Management Liabilities Current

Derivatives, Fair Value [Line Items]

Derivative Liability

^[3]

0.9

1.5

Commodity Price Risk Programs | Risk Management Liabilities Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Liability

$ 32.3

$ 20.0

[1]	Presented in "Prepayments and other" on the Condensed Consolidated Balance Sheets (unaudited).
[2]	Presented in "Deferred charges and other" on the Condensed Consolidated Balance Sheets (unaudited).
[3]	Presented in "Other accruals" on the Condensed Consolidated Balance Sheets (unaudited).

X

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

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X

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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- Definition

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+ References

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