EQUITY METHOD INVESTMENTS - CHANGES TO EQUITY METHOD INVESTMENTS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Changes to equity method investments
Equity method investment balance at beginning of period	$ 540.9us-gaap_EquityMethodInvestments
Earnings from equity method investments	88.3	91.5	87.2
Equity method investment balance at end of period	572.4us-gaap_EquityMethodInvestments	540.9us-gaap_EquityMethodInvestments
ATC
Equity method investments
Equity method investment, ownership interest (as a percent)	34.00%
Changes to equity method investments
Equity method investment balance at beginning of period	508.4	476.6	439.4
Earnings from equity method investments	85.7	89.1	85.3
Add: Capital contributions	17.0	13.7	20.4
Less: Dividends received	74.4	71.0	68.5
Equity method investment balance at end of period	536.7	508.4	476.6
ATC \| Return on equity reduction for MISO transmission owners
Equity method investments
Requested return on equity (as a percent)	9.15%
Authorized return on equity (as a percent)	12.20%
Reduction in equity method investment earnings	6.6
INDU Solar Holdings, LLC
Equity method investments
Equity method investment, ownership interest (as a percent)	50.00%
Changes to equity method investments
Equity method investment balance at beginning of period	24.7	27.5	28.4
Earnings from equity method investments	1.8	1.3	1.1
Add: Capital contributions	0	0	7.0
Less: Return of capital to partners	4.7	4.1	9.0
Equity method investment balance at end of period	21.8	24.7	27.5
WRPC
Equity method investments
Equity method investment, ownership interest (as a percent)	50.00%
Changes to equity method investments
Equity method investment balance at beginning of period	7.0	7.3	7.7
Earnings from equity method investments	0.8	1.0	0.8
Add: Capital contributions	0.5	0	0
Less: Dividends received	0.6	1.3	1.2
Equity method investment balance at end of period	7.7	7.0	7.3
AMP Trillium, LLC
Changes to equity method investments
Equity method investment balance at beginning of period	0
Add: Capital contributions	5.5
Equity method investment balance at end of period	$ 5.5
AMP Trillium, LLC \| AMP Americas, LLC
Equity method investments
Equity method investment, ownership interest (as a percent)	70.00%
AMP Trillium, LLC \| ITF
Equity method investments
Equity method investment, ownership interest (as a percent)	30.00%

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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- Definition

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

+ References

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-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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- Definition

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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Total change in each class of partners' capital accounts during the year due to the return of partner's capital. That is, excess distributions over earnings to partners. All partners include general, limited and preferred partners.

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- Definition

Percentage of public utility's approved return on equity.

+ References

No definition available.

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Percentage of public utility's requested return on equity.

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