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Summary of Significant Accounting Policies Materials, Supplies, and Inventories (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / Dekatherm	Dec. 31, 2016 USD ($) $ / Dekatherm
Inventory
Natural gas in storage	$ 209.0	$ 223.1
Materials and supplies	211.2	206.5
Fossil fuel	118.8	158.0
Total	$ 539.0	$ 587.6
Percentage of LIFO inventory	15.00%	18.00%
PGL and NSG
Inventory
Excess of replacement or current costs over stated LIFO value	$ 152.1	$ 92.9
Natural gas price benchmark | $ / Dekatherm	4.68	3.63

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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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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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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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