RISK MANAGEMENT ACTIVITIES - INTEGRYS ENERGY SERVICES SEGMENT, GAINS AND LOSSES (Details) (Integrys Energy Services, USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (total)	$ 22.2	$ (70.1)	$ 179.2	$ (5.4)
Natural gas contracts | Nonregulated revenue
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	10.0	33.8	(26.9)	37.2
Natural gas contracts | Nonregulated cost of sales
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	(5.0)	(32.9)	28.0	(34.5)
Natural gas contracts | Nonregulated revenue (reclassified from accumulated OCI)
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (prior cash flow hedge)	0	(0.1)	0	(0.2)
Electric contracts | Nonregulated revenue
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	15.8	(77.6)	176.1	(13.6)
Electric contracts | Nonregulated cost of sales
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	1.4	8.7	2.0	8.7
Electric contracts | Nonregulated revenue (reclassified from accumulated OCI)
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (prior cash flow hedge)	$ 0	$ (2.0)	$ 0	$ (3.0)