Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 7.4	$ 7.6
Regulatory Asset, Current	71.9	22.6
Regulatory Asset, Noncurrent	224.4	285.5
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current	35.2	16.0
Regulatory Asset, Noncurrent	107.9	135.2
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current	11.8	5.5
Regulatory Asset, Noncurrent	69.8	85.1
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current	0.9	0.8
Regulatory Asset, Noncurrent	18.6	16.8
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current	0.0	0.0
Regulatory Asset, Noncurrent	17.1	16.9
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current	0.2	0.2
Regulatory Asset, Noncurrent	2.3	2.4
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory Asset, Current	23.4	0.0
Regulatory Asset, Noncurrent	0.0	25.2
Other
Regulatory Assets [Line Items]
Regulatory Asset, Current	0.4	0.1
Regulatory Asset, Noncurrent	$ 8.7	$ 3.9

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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- Definition

Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 55
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82849221&loc=d3e44360-110383

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