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MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Inventory Disclosure [Abstract] |  |
Natural gas in storage | $ 121.9 | $ 209.0
Materials and supplies | 221.4 | 211.2
Fossil fuel | 123.3 | 118.8
Total | 466.6 | $ 539.0
LIFO cost method |  |
LIFO liquidation balance | $ 11.1 |

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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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