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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
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Inventory Disclosure [Abstract] |  |
Natural gas in storage | $ 301.5 | $ 223.1
Materials and supplies | 225.1 | 206.5
Fossil fuel | 145.6 | 158.0
Total | 672.2 | $ 587.6
LIFO cost method |  |
LIFO liquidation balance | $ 0.0 |

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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