FAIR VALUE - LEVEL 3 RECONCILIATION (Details) (Utility segments, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	$ (1.3)	$ (5.4)	$ (5.7)
Net realized and unrealized gains included in earnings	0.2	3.0	1.8
Net unrealized (losses) gains recorded as regulatory assets or liabilities	(7.8)	0.3	5.7
Purchases	4.3	3.2	2.8
Sales		(0.2)	(0.1)
Settlements	(3.5)	(2.2)	(9.9)
Net transfers out of Level 3	1.2
Balance at the end of the period	(6.9)	(1.3)	(5.4)
Impact on earnings of unrealized gains (losses) on level 3 instruments	0	0	0
Natural gas contracts
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	0
Net realized and unrealized gains included in earnings	0
Net unrealized (losses) gains recorded as regulatory assets or liabilities	(6.6)
Purchases	0
Settlements	0
Net transfers out of Level 3	0
Balance at the end of the period	(6.6)
FTRs
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	1.2	1.1	1.2
Net realized and unrealized gains included in earnings	0.2	3.0	1.8
Net unrealized (losses) gains recorded as regulatory assets or liabilities	0.4	(0.1)	(0.1)
Purchases	4.3	3.2	2.8
Sales		(0.2)	(0.1)
Settlements	(4.2)	(5.8)	(4.5)
Net transfers out of Level 3	0
Balance at the end of the period	1.9	1.2	1.1
Coal contracts
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Balance at the beginning of the period	(2.5)	(6.5)	(6.9)
Net realized and unrealized gains included in earnings	0	0	0
Net unrealized (losses) gains recorded as regulatory assets or liabilities	(1.6)	0.4	5.8
Purchases	0	0	0
Sales		0	0
Settlements	0.7	3.6	(5.4)
Net transfers out of Level 3	1.2
Balance at the end of the period	$ (2.2)	$ (2.5)	$ (6.5)