Summary of Significant Accounting Policies Materials, Supplies, and Inventories (Details) $ in Millions	Dec. 31, 2019 USD ($) $ / Dekatherm	Dec. 31, 2018 USD ($) $ / Dekatherm
Inventory
Materials and supplies	$ 234.2	$ 226.6
Natural gas in storage	227.7	232.9
Fossil fuel	87.9	88.7
Total	$ 549.8	$ 548.2
Percentage of LIFO inventory	19.00%	16.00%
PGL and NSG
Inventory
Excess of replacement or current costs over stated LIFO value	$ 9.8	$ 72.4
Natural gas price benchmark \| $ / Dekatherm	1.95	3.08

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.

+ References

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Amount of fossil fuel included in inventory classified as other.

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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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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.

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No definition available.

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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.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 210
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-Section S99
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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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Represents the Chicago city-gate natural gas price used in calculating replacement amounts.

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