Investment in ATC - Changes to Investment in ATC (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 29, 2015
Investment in ATC
Equity method investment, ownership interest (as a percent)					60.00%
Changes to investment in ATC
Investment in ATC, balance at beginning of period			$ 424.1
Add: Earnings from equity method investment	$ 40.0	$ 18.0	70.4	$ 52.8
Investment in ATC, balance at end of period	$ 999.4		$ 999.4
ATC
Investment in ATC
Equity method investment, ownership interest (as a percent)	60.00%		60.00%		26.20%
Changes to investment in ATC
Investment in ATC, balance at beginning of period	$ 987.8	416.8	$ 424.1	402.7
Add: Earnings from equity method investment	40.0	18.0	70.4	52.8
Add: Capital contributions	3.0	2.6	5.5	10.4
Add: Acquisition of Integrys's investment in ATC	0.0	0.0	552.0	0.0
Less: Distributions received	31.4	14.1	52.6	42.6
Investment in ATC, balance at end of period	$ 999.4	$ 423.3	$ 999.4	$ 423.3

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.

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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

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-Publisher FASB

-Name Accounting Standards Codification

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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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 45

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-Section S99

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-Subparagraph (SX 210.5-02.12)

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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.

+ References

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Investment in common stock of an equity method investee acquired as part of a business combination.

+ References

No definition available.

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

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No definition available.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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