EQUITY METHOD INVESTMENTS - COMBINED FINANCIAL DATA (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income statement data
Earnings from equity method investments	$ 88.3	$ 91.5	$ 87.2
ATC, INDU Solar Holdings, LLC, WRPC, and AMP Trillium, LLC
Income statement data
Revenues	655.6	642.0	618.3
Operating expenses	323.5	306.2	292.1
Other expense	88.4	83.7	85.1
Net income	243.7	252.1	241.1
Earnings from equity method investments	88.3	91.4	87.2
Balance sheet data
Current assets	80.7	90.2
Noncurrent assets	3,835.9	3,587.2
Total assets	3,916.6	3,677.4
Current liabilities	324.0	383.6
Long-term debt	1,721.6	1,559.1
Other noncurrent liabilities	173.2	134.4
Shareholders' equity	1,697.8	1,600.3
Total liabilities and shareholders' equity	$ 3,916.6	$ 3,677.4

The amount of long-term debt reported by an equity method investment of the entity.

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The amount of other expenses reported by an equity method investment of the entity, which are not separately shown.

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No definition available.

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The amount of other noncurrent liabilities reported by an equity method investment of the entity.

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No definition available.

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The amount of assets reported by an equity method investment of the entity.

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- Definition

The amount of the cost of sales reported by an equity method investment of the entity.

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The amount of current assets reported by an equity method investment of the entity.

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The amount of current liabilities reported by an equity method investment of the entity.

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Amount of equity, excluding noncontrolling interest, attributable to the equity method investment of the entity.

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The amount of liabilities and equity reported by an equity method investment of the entity.

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The amount of net income (loss) reported by an equity method investment of the entity.

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The amount of noncurrent assets reported by an equity method investment of the entity.

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The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

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No definition available.

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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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