EQUITY METHOD INVESTMENTS (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Changes in equity method investments
Equity method investments balance at beginning of period	$ 512.2
Earnings from equity method investments	91.5	87.2	79.4
Add: Capital contributions	13.7	27.4	37.6
Equity method investments balance at end of period	540.9	512.2
ATC
Investments in affiliates, at equity method
Equity method investment, ownership interest (as a percent)	34.00%
Changes in equity method investments
Equity method investments balance at beginning of period	476.6	439.4	416.3
Earnings from equity method investments	89.1	85.3	79.1
Add: Capital contributions	13.7	20.4	8.5
Less: Dividends received	71.0	68.5	64.5
Equity method investments balance at end of period	508.4	476.6	439.4
INDU Solar Holdings, LLC
Investments in affiliates, at equity method
Equity method investment, ownership interest (as a percent)	50.00%
Changes in equity method investments
Equity method investments balance at beginning of period	27.5	28.4	0.1
Earnings from equity method investments	1.3	1.1	(0.7)
Add: Capital contributions	0	7.0	29.0
Less: Return of capital to partners	4.1	9.0	0
Equity method investments balance at end of period	24.7	27.5	28.4
WRPC
Investments in affiliates, at equity method
Equity method investment, ownership interest (as a percent)	50.00%
Changes in equity method investments
Equity method investments balance at beginning of period	7.3	7.7	8.1
Earnings from equity method investments	1.0	0.8	0.9
Less: Dividends received	1.3	1.2	1.3
Equity method investments balance at end of period	$ 7.0	$ 7.3	$ 7.7