FAIR VALUE (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Calculation
Balance at the beginning of the period	$ (4.9)	$ (7.9)
Net realized and unrealized (losses) gains included in earnings	13.9	(3.1)
Net unrealized (losses) gains recorded as regulatory assets or liabilities	2.9	(5.7)
Purchases	0.7	1.1
Sales		(0.1)
Settlements	(3.2)	(4.0)
Net transfers into Level 3	0.2	(2.6)
Net transfers out of Level 3	(5.5)	(1.2)
Balance at the end of the period	4.1	(23.5)
Net unrealized (losses) gains included in earnings related to instruments still held at the end of the period	13.6	(3.6)
Nonregulated Segments | Natural gas contracts
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Calculation
Balance at the beginning of the period	3.9	8.3
Net realized and unrealized (losses) gains included in earnings	(1.5)	4.1
Settlements	(0.9)	(2.6)
Net transfers into Level 3	0.2	2.4
Net transfers out of Level 3		(1.3)
Balance at the end of the period	1.7	10.9
Net unrealized (losses) gains included in earnings related to instruments still held at the end of the period	(1.5)	4.1
Nonregulated Segments | Electric contracts
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Calculation
Balance at the beginning of the period	(4.3)	(11.5)
Net realized and unrealized (losses) gains included in earnings	15.1	(7.7)
Purchases	0.7	1.1
Settlements	0.1	1.1
Net transfers into Level 3		(5.0)
Net transfers out of Level 3	(5.5)	0.1
Balance at the end of the period	6.1	(21.9)
Net unrealized (losses) gains included in earnings related to instruments still held at the end of the period	15.1	(7.7)
Utility Segments | FTRs
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Calculation
Balance at the beginning of the period	2.0	2.2
Net realized and unrealized (losses) gains included in earnings	0.3	0.5
Net unrealized (losses) gains recorded as regulatory assets or liabilities	(0.2)	0.1
Sales		(0.1)
Settlements	(1.2)	(1.8)
Balance at the end of the period	0.9	0.9
Utility Segments | Coal contract
Fair Value Assets and Liabilities Measured on Recurring Basis Unobservable Input Reconciliation Calculation
Balance at the beginning of the period	(6.5)	(6.9)
Net unrealized (losses) gains recorded as regulatory assets or liabilities	3.1	(5.8)
Settlements	(1.2)	(0.7)
Balance at the end of the period	$ (4.6)	$ (13.4)