FAIR VALUE - SIGNIFICANT UNOBSERVABLE INPUTS (Details) (USD $) In Millions, unless otherwise specified					9 Months Ended			9 Months Ended				9 Months Ended					9 Months Ended			9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Fair value measurements on a recurring basis Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Level 3	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Valuation Technique: Market-based	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Natural gas contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Natural gas contracts	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Natural gas contracts Valuation Technique: Market-based Minimum	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Natural gas contracts Valuation Technique: Market-based Maximum	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Electric contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Electric contracts	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Electric contracts Valuation Technique: Market-based	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Electric contracts Valuation Technique: Market-based Minimum	Sep. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3 Electric contracts Valuation Technique: Market-based Maximum	Sep. 30, 2014 Fair value measurements on a recurring basis Utility segments Level 3 FTRs	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Level 3 FTRs	Sep. 30, 2013 Fair value measurements on a recurring basis Utility segments Level 3 FTRs Valuation Technique: Market-based Weighted Average	Sep. 30, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts	Sep. 30, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts Valuation Technique: Market-based Minimum	Sep. 30, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Coal contracts Valuation Technique: Market-based Maximum	Sep. 30, 2014 Fair value measurements on a recurring basis Utility segments Level 3 Natural gas contracts	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Level 3 Natural gas contracts
Significant internally-developed unobservable inputs
Risk Management Assets	$ 340.9	$ 315.5	$ 45.8	$ 53.8		$ 27.6	$ 35.6			$ 12.4	$ 15.9				$ 3.4	$ 2.1		$ 2.4	$ 0.2			$ 0	$ 0
Risk Management Liabilities	$ 235.9	$ 226.6	$ 23.4	$ 31.7		$ 16.7	$ 25.2			$ 3.9	$ 3.5				$ 0.4	$ 0.3		$ 2.4	$ 2.7			$ 0	$ 0
Forward market prices								(1.94)	7.71				(3.00)	12.10			187.89			12.31	15.50
Probability of default (as a percent)								11.60%	51.00%			26.00%
Option volatilities (as a percent)													18.70%	116.00%
Counterparty default time period					1 year