MATERIALS, SUPPLIES, AND INVENTORIES (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Natural gas in storage	$ 34.8	$ 209.0
Materials and supplies	213.7	211.2
Fossil fuel	127.5	118.8
Total	376.0	$ 539.0
LIFO cost method
LIFO liquidation balance	$ 35.6

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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- Definition

Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale.

+ References

+ Details

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- Definition

Amount of fossil fuel included in inventory classified as other.

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+ Details

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- Definition

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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+ Details

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- References

No definition available.

+ Details

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- Definition

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.

+ 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)(4))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 330
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 5.BB)
-URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100047-122729

Reference 3: 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)(5))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- References

No definition available.

+ Details

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