RISK MANAGEMENT ACTIVITIES (Details 5) (Nonregulated Segments, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash flow hedges
Risk management activities
Unrealized Gain (Loss) Recognized in OCI of Derivative Instruments (Effective Portion)	$ 1.5	$ (34.8)	$ (56.8)
Gain (loss) Reclassified from AOCI into income	(6.7)	(47.5)	(103.4)
Gain (Loss) Recognized in Income on Derivative Instruments (Ineffective Portion and Amount Excluded from Effectiveness Testing)		(1.6)
Cash flow hedges | Natural gas contracts
Risk management activities
Unrealized Gain (Loss) Recognized in OCI of Derivative Instruments (Effective Portion)	(2.3)	(15.2)
Cash flow hedges | Natural gas contracts | Nonregulated revenue
Risk management activities
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(9.3)	(16.4)
Gain (Loss) Reclassified from Accumulated OCI into Income (Hedge Designation Discontinued)	(0.3)	0.2
Gain (Loss) Recognized in Income on Derivative Instruments (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0.3	(1.1)
Cash flow hedges | Electric contracts
Risk management activities
Unrealized Gain (Loss) Recognized in OCI of Derivative Instruments (Effective Portion)	3.8	(13.6)
Cash flow hedges | Electric contracts | Nonregulated revenue
Risk management activities
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	4.2	(21.6)
Gain (Loss) Reclassified from Accumulated OCI into Income (Hedge Designation Discontinued)		(9.9)
Gain (Loss) Recognized in Income on Derivative Instruments (Ineffective Portion and Amount Excluded from Effectiveness Testing)	(0.3)	(0.5)
Cash flow hedges | Commodity contracts
Risk management activities
Unrealized Gain (Loss) Recognized in OCI of Derivative Instruments (Effective Portion)			(60.0)
Cash flow hedges | Commodity contracts | Nonregulated revenue
Risk management activities
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			(107.3)
Gain (Loss) Reclassified from Accumulated OCI into Income (Hedge Designation Discontinued)			2.7
Gain (Loss) Recognized in Income on Derivative Instruments (Ineffective Portion and Amount Excluded from Effectiveness Testing)			(1.1)
Cash flow hedges | Interest rate swaps
Risk management activities
Unrealized Gain (Loss) Recognized in OCI of Derivative Instruments (Effective Portion)		(6.0)	3.2
Cash flow hedges | Interest rate swaps | Interest expense.
Risk management activities
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.1)	0.2	1.2
Gain (Loss) Reclassified from Accumulated OCI into Income (Hedge Designation Discontinued)	$ (0.2)
Purchases | Natural gas contracts
Risk management activities
Notional volume of outstanding cash flow hedges		265,600,000
Purchases | Electric contracts
Risk management activities
Notional volume of outstanding cash flow hedges		11,569,000,000
Sales | Electric contracts
Risk management activities
Notional volume of outstanding cash flow hedges		29,800,000