REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Regulatory liabilities
Current liabilities	$ 153.7us-gaap_RegulatoryLiabilityCurrent	$ 101.1us-gaap_RegulatoryLiabilityCurrent
Long-term liabilties	399.9us-gaap_RegulatoryLiabilityNoncurrent	383.7us-gaap_RegulatoryLiabilityNoncurrent
Total regulatory liabilities	553.6us-gaap_RegulatoryLiabilities	484.8us-gaap_RegulatoryLiabilities
Removal costs
Regulatory liabilities
Total regulatory liabilities	334.0	318.0
Decoupling
Regulatory liabilities
Total regulatory liabilities	49.4	51.5
Unrecognized pension and other postretirement benefit costs
Regulatory liabilities
Total regulatory liabilities	45.2	30.2
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	44.8	27.1
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	21.3	19.6
Derivatives
Regulatory liabilities
Total regulatory liabilities	19.8	6.6
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	15.7	10.1
Crane Creek depreciation deferral
Regulatory liabilities
Total regulatory liabilities	8.7	9.0
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	22.8	22.1
Other
Regulatory liabilities
Total regulatory liabilities	$ 10.1	$ 7.1

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 current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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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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