RISK MANAGEMENT ACTIVITIES - INTEGRYS ENERGY SERVICES SEGMENT, GAINS AND LOSSES (Details) (Integrys Energy Services, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (total)	$ 9.5	$ 33.1	$ 188.7	$ 27.7
Natural gas contracts | Nonregulated revenue
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	25.9	(21.1)	(1.0)	16.1
Natural gas contracts | Nonregulated cost of sales
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	(20.5)	25.0	7.5	(9.5)
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	0	(0.2)
Electric contracts | Nonregulated revenue
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	4.1	36.0	180.2	22.4
Electric contracts | Nonregulated cost of sales
Risk management activities
Gains (losses) recorded related to non-hedge derivatives	0	(6.6)	2.0	2.1
Electric contracts | Nonregulated revenue (reclassified from accumulated OCI)
Risk management activities
Gains (losses) recorded related to non-hedge derivatives (prior cash flow hedge)	$ 0	$ (0.2)	$ 0	$ (3.2)