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Summary of Significant Accounting Policies Materials, Supplies, and Inventories (Details) $ in Millions	Dec. 31, 2018 USD ($) $ / Dekatherm	Dec. 31, 2017 USD ($) $ / Dekatherm
Inventory
Natural gas in storage	$ 232.9	$ 209.0
Materials and supplies	226.6	211.2
Fossil fuel	88.7	118.8
Total	$ 548.2	$ 539.0
Percentage of LIFO inventory	16.00%	15.00%
PGL and NSG
Inventory
Excess of replacement or current costs over stated LIFO value	$ 72.4	$ 152.1
Natural gas price benchmark | $ / Dekatherm	3.08	4.68

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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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