FAIR VALUE - LEVEL 3 RECONCILIATION (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
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	$ (5.6)	$ 0.8	$ (6.9)	$ (1.3)
Net realized gains (losses) included in earnings	(1.2)	0.1	(2.4)	0.8
Net unrealized gains (losses) recorded as regulatory assets or liabilities	(4.5)	0.9	(9.0)	3.1
Purchases	9.8	4.4	9.8	4.3
Settlements	0.2	(1.3)	7.2	(2.0)
Balance at the end of the period	(1.3)	4.9	(1.3)	4.9
Impact on earnings of unrealized gains (losses) on level 3 instruments	0.0	0.0	0.0	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			(6.6)
Net realized gains (losses) included in earnings			0.0
Net unrealized gains (losses) recorded as regulatory assets or liabilities			0.0
Purchases			0.0
Settlements			6.6
Balance at the end of the period	0.0		0.0
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	0.6	0.5	1.9	1.2
Net realized gains (losses) included in earnings	(1.2)	0.1	(2.4)	0.8
Net unrealized gains (losses) recorded as regulatory assets or liabilities	(4.8)	0.1	(5.0)	0.1
Purchases	9.8	4.4	9.8	4.3
Settlements	(0.3)	(1.1)	(0.2)	(2.4)
Balance at the end of the period	4.1	4.0	4.1	4.0
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	(6.2)	0.3	(2.2)	(2.5)
Net realized gains (losses) included in earnings	0.0	0.0	0.0	0.0
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0.3	0.8	(4.0)	3.0
Purchases	0.0	0.0	0.0	0.0
Settlements	0.5	(0.2)	0.8	0.4
Balance at the end of the period	$ (5.4)	$ 0.9	$ (5.4)	$ 0.9