INVESTMENT IN ATC - INVESTMENT ROLL FORWARD (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Changes to investment in ATC
Investment in ATC, balance at beginning of period	$ 550.6us-gaap_EquityMethodInvestments
Add: Earnings from equity method investment	17.5	22.9
Investment in ATC, balance at end of period	556.0us-gaap_EquityMethodInvestments
ATC
Investment in ATC
Equity method investment, ownership interest (as a percent)	34.00%
Changes to investment in ATC
Investment in ATC, balance at beginning of period	536.7	508.4
Add: Earnings from equity method investment	17.0	22.5
Add: Capital contributions	1.7	5.1
Less: Dividends received	13.6	18.4
Investment in ATC, balance at end of period	541.8	517.6
ATC \| Return on equity reduction for MISO transmission owners
Investment in ATC
Requested return on equity (as a percent)	9.15%
Authorized return on equity (as a percent)	12.20%
Reduction in ATC equity earnings related to prior years	$ 4.8

Cash or noncash contributions of capital made by the entity to an equity method investee.

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This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities.

+ References

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

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

-Publisher FASB

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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

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This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

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The amount of loss pertaining to the specified contingency that was charged against earnings in the period, including the effects of revisions in previously reported estimates.

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Percentage of public utility's approved return on equity.

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No definition available.

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Percentage of public utility's requested return on equity.

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No definition available.

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