Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.3	$ 0.4
Total Derivative Assets	0.0	0.0
Total Assets	0.3	0.4
Total Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	8.5	7.7
Total Derivative Assets	0.1	1.5
Total Assets	8.6	9.2
Total Liabilities	1,386.5	1,501.9
Fair Value, Inputs, Level 2 [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	0.1	1.5
Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Total Derivative Assets	0.0	0.0
Total Assets	0.0	0.0
Total Liabilities	17.5	17.7
Fair Value, Inputs, Level 3 [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Equity Securities [Member] \| Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	4.2	3.5
Equity Securities [Member] \| Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	4.1	4.0
Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Money Market Funds [Member] \| Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] \| Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] \| Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Hedge Funds [Member] \| Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] \| Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] \| Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Tangible Assets [Member] \| Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt [Member] \| Fair Value, Inputs, Level 1 [Member]
Debt Instrument, Fair Value Disclosure	0.0	0.0
Debt [Member] \| Fair Value, Inputs, Level 2 [Member]
Debt Instrument, Fair Value Disclosure	1,386.5	1,501.9
Debt [Member] \| Fair Value, Inputs, Level 3 [Member]
Debt Instrument, Fair Value Disclosure	17.5	17.7
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.8	8.1
Total Derivative Assets	0.1	1.5
Total Assets	8.9	9.6
Total Liabilities	1,404.0	1,519.6
Fair Value [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	0.1	1.5
Fair Value [Member] \| Equity Securities [Member]
Total Master Trust Assets, Fair Value	4.2	3.5
Fair Value [Member] \| Debt Securities [Member]
Total Master Trust Assets, Fair Value	4.1	4.0
Fair Value [Member] \| Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Fair Value [Member] \| Hedge Funds [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Fair Value [Member] \| Tangible Assets [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Fair Value [Member] \| Debt [Member]
Debt Instrument, Fair Value Disclosure	$ 1,404.0	$ 1,519.6

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: 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

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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
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-Section 50
-Paragraph 3
-Subparagraph (c)
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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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- Definition

Fair value of financial and nonfinancial obligations.

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