REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory Liabilities
Current liabilities	$ 21.2us-gaap_RegulatoryLiabilityCurrent	$ 38.0us-gaap_RegulatoryLiabilityCurrent
Long-term liabilities	303.3us-gaap_RegulatoryLiabilityNoncurrent	286.3us-gaap_RegulatoryLiabilityNoncurrent
Total regulatory liabilities	324.5us-gaap_RegulatoryLiabilities	324.3us-gaap_RegulatoryLiabilities
Removal costs
Regulatory Liabilities
Total regulatory liabilities	243.9	238.0
Unrecognized pension and other postretirement benefit costs
Regulatory Liabilities
Total regulatory liabilities	42.4	18.5
Decoupling
Regulatory Liabilities
Total regulatory liabilities	12.3	24.3
Crane Creek depreciation deferral
Regulatory Liabilities
Total regulatory liabilities	8.7	9.0
Energy costs refundable through rate adjustments
Regulatory Liabilities
Total regulatory liabilities	6.0	21.9
Fox Energy Center
Regulatory Liabilities
Total regulatory liabilities	4.6	5.6
Other Disclosures
Amount of allowance for earnings on equity capitalized for rate making purposes not included in the deferral	22.8	22.1
Energy efficiency programs
Regulatory Liabilities
Total regulatory liabilities	3.7	4.1
Other
Regulatory Liabilities
Total regulatory liabilities	$ 2.9	$ 2.9

Discloses the amount of any allowance for earnings on shareholders' investment capitalized for rate making purposes but not for the financial statements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 3

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 405

-Section 25

-Paragraph 1

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ 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 210.5-02.24)

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