FAIR VALUE - LEVEL 3 RECONCILIATION (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
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	$ 14.3	$ (4.9)
Net realized and unrealized gains (losses) included in earnings	6.8	13.9
Net unrealized gains (losses) recorded as regulatory assets or liabilities	2.2	2.9
Purchases	0.6	0.7
Sales	(0.7)
Settlements	(4.8)	(3.2)
Net transfers into Level 3	3.5	0.2
Net transfers out of Level 3	(4.5)	(5.5)
Balance at the end of the period	17.4	4.1
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	6.4	13.6
Nonregulated segments | Contingent consideration
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	(7.8)
Net realized and unrealized gains (losses) included in earnings	0
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0
Purchases	0
Sales	0
Settlements	0
Net transfers into Level 3	0
Net transfers out of Level 3	0
Balance at the end of the period	(7.8)
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	0
Nonregulated segments | 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	10.4	3.9
Net realized and unrealized gains (losses) included in earnings	(6.2)	(1.5)
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0	0
Purchases	0	0
Sales	0
Settlements	0.5	(0.9)
Net transfers into Level 3	0.9	0.2
Net transfers out of Level 3	(0.1)	0
Balance at the end of the period	5.5	1.7
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	(6.2)	(1.5)
Nonregulated segments | Electric 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	12.4	(4.3)
Net realized and unrealized gains (losses) included in earnings	12.6	15.1
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0	0
Purchases	0.7	0.7
Sales	(0.7)
Settlements	(4.5)	0.1
Net transfers into Level 3	2.6	0
Net transfers out of Level 3	(4.4)	(5.5)
Balance at the end of the period	18.7	6.1
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	12.6	15.1
Utility segments
Changes in the fair value of items measured on a recurring basis and categorized as Level 3 measurements
Impact on earnings of unrealized gains (losses) on level 3 instruments	0	0
Utility segments | 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	1.8	2.0
Net realized and unrealized gains (losses) included in earnings	0.4	0.3
Net unrealized gains (losses) recorded as regulatory assets or liabilities	0	(0.2)
Purchases	(0.1)	0
Sales	0
Settlements	(1.4)	(1.2)
Net transfers into Level 3	0	0
Net transfers out of Level 3	0	0
Balance at the end of the period	0.7	0.9
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	0	0
Utility segments | 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)
Net realized and unrealized gains (losses) included in earnings	0	0
Net unrealized gains (losses) recorded as regulatory assets or liabilities	2.2	3.1
Purchases	0	0
Sales	0
Settlements	0.6	(1.2)
Net transfers into Level 3	0	0
Net transfers out of Level 3	0	0
Balance at the end of the period	0.3	(4.6)
Net unrealized gains (losses) included in earnings related to instruments still held at the end of the period	$ 0	$ 0