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

Dec. 31, 2018

Dec. 31, 2017

Derivatives, Fair Value [Line Items]

Derivative Asset

$ 24.0

$ 21.1

Derivative Liability

51.7

71.7

Risk Management Assets Current

Derivatives, Fair Value [Line Items]

Derivative Asset

^[1]

1.1

14.5

Risk Management Assets Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Asset

^[2]

22.9

6.6

Risk Management Liabilities Current

Derivatives, Fair Value [Line Items]

Derivative Liability

5.0

43.2

Risk Management Liabilities Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Liability

46.7

28.5

Interest rate risk programs | Risk Management Assets Current

Derivatives, Fair Value [Line Items]

Derivative Asset

0.0

14.0

Interest rate risk programs | Risk Management Assets Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Asset

18.5

5.6

Interest rate risk programs | Risk Management Liabilities Current

Derivatives, Fair Value [Line Items]

Derivative Liability

0.0

38.6

Interest rate risk programs | Risk Management Liabilities Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Liability

9.5

0.0

Commodity price risk programs | Risk Management Assets Current

Derivatives, Fair Value [Line Items]

Derivative Asset

1.1

0.5

Commodity price risk programs | Risk Management Assets Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Asset

4.4

1.0

Commodity price risk programs | Risk Management Liabilities Current

Derivatives, Fair Value [Line Items]

Derivative Liability

5.0

4.6

Commodity price risk programs | Risk Management Liabilities Noncurrent

Derivatives, Fair Value [Line Items]

Derivative Liability

$ 37.2

$ 28.5

[1]	Presented in "Prepayments and other" on the Consolidated Balance Sheets.
[2]	Presented in "Deferred charges and other" on the Consolidated Balance Sheets.

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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

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- 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://fasb.org/us-gaap/role/ref/legacyRef
-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/disclosureRef
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-Name Accounting Standards Codification
-Topic 825
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
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-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

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- Definition

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