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Summary of Significant Accounting Policies Materials, Supplies, and Inventories (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 USD ($) $ / Dekatherm | Dec. 31, 2015 USD ($) $ / Dekatherm
Inventory |  |
Natural gas in storage | $ 223.1 | $ 284.1
Materials and supplies | 206.5 | 219.2
Fossil fuel | 158.0 | 183.7
Total | $ 587.6 | $ 687.0
Percentage of LIFO inventory | 18.00% | 18.00%
PGL and NSG |  |
Inventory |  |
Excess of replacement or current costs over stated LIFO value | $ 92.9 | $ 15.2
Natural gas price benchmark | $ / Dekatherm | 3.63 | 2.48

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