Nature of Operations And Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016
Basis Of Accounting Presentation [Line Items]
Number of customers	4,000,000
Unbilled revenue, less reserves	$ 324.2	$ 359.4
Other than Temporary Impairment Losses, Investments	$ 0.0	$ 0.0
Pre-tax rate for allowance for funds used during construction	3.50%	4.00%	4.50%
Inventory valued using LIFO	$ 47.5	$ 45.5
Excess of replacement over LIFO value	(12.2)	(17.4)
Inventory valued using the weighted average cost methodology	$ 239.3	$ 239.6

Basis Of Accounting Presentation [Line Items]

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Number of customers

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- Definition

Pre tax rate for allowance for funds used during construction

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- Definition

The excess of replacement or current cost over the stated LIFO (last in first out) inventory value when the LIFO inventory method is utilized.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Paragraph 1
-Subparagraph (SX 210.5-02.6(c))
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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
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The amount of LIFO (last in first out) inventory present at the reporting date when inventory is also valued using different valuation methods.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 50
-Paragraph 1
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Amount received for services rendered and products shipped, but not yet billed, for non-contractual agreements due within one year or the normal operating cycle, if longer.

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The amount of weighted average cost inventory present at the reporting date when inventory is valued using different valuation methods.

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