Summary of Significant Accounting Policies Materials, Supplies, and Inventories (Details) $ in Millions	Dec. 31, 2023 USD ($) $ / MMBTU	Dec. 31, 2022 USD ($) $ / MMBTU
Accounting Policies [Abstract]
Natural gas in storage	$ 327.8	$ 446.3
Materials and supplies	320.0	257.0
Fossil fuel	127.4	103.8
Total	$ 775.2	$ 807.1
LIFO Method Related Items [Abstract]
Percentage of LIFO inventory	17.00%	13.00%
Excess of replacement or current costs over stated LIFO value	$ 12.2	$ 98.3
Natural gas price benchmark \| $ / MMBTU	2.13	3.41

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Carrying amount as of the balance sheet date of merchandise, goods, commodities, or supplies relating to a source of usable power held for future sale or future use in manufacturing or production process.

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Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale.

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Amount of fossil fuel included in inventory classified as other.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(6)(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

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- Definition

Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed. This amount is net of valuation reserves and adjustments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(6))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 330
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.BB)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480581/330-10-S99-2

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The percentage of LIFO (last in first out) inventory to total inventory as of the balance sheet date if other than 100 percent.

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No definition available.

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Represents the Chicago city-gate natural gas price used in calculating replacement amounts.

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