FAIR VALUE - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) (USD $) In Millions, unless otherwise specified																																																																											1 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Fair value measurements on a recurring basis Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Natural gas contracts Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Natural gas contracts Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Natural gas contracts Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Natural gas contracts Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Natural gas contracts Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Natural gas contracts Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Natural gas contracts Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Natural gas contracts Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments FTRs Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments FTRs Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments FTRs Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments FTRs Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments FTRs Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments FTRs Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments FTRs Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments FTRs Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Petroleum product contracts Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Coal contracts Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Coal contracts Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Coal contracts Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Coal contracts Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Coal contracts Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Coal contracts Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Utility segments Coal contracts Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Utility segments Coal contracts Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Natural gas contracts Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Integrys Energy Services Electric contracts Total, Fair value	Jun. 30, 2014 Fair value measurements on a recurring basis Holding Company and Other Level 1	Dec. 31, 2013 Fair value measurements on a recurring basis Holding Company and Other Level 1	Jun. 30, 2014 Fair value measurements on a recurring basis Holding Company and Other Level 2	Dec. 31, 2013 Fair value measurements on a recurring basis Holding Company and Other Level 2	Jun. 30, 2014 Fair value measurements on a recurring basis Holding Company and Other Level 3	Dec. 31, 2013 Fair value measurements on a recurring basis Holding Company and Other Level 3	Jun. 30, 2014 Fair value measurements on a recurring basis Holding Company and Other Total, Fair value	Dec. 31, 2013 Fair value measurements on a recurring basis Holding Company and Other Total, Fair value	Jul. 31, 2014 Subsequent Event Fair value measurements on a recurring basis Integrys Energy Services Level 3
Assets
Risk Management Assets	$ 342.9	$ 315.5	$ 120.6	$ 83.9	$ 164.3	$ 177.8	$ 58.0	$ 53.8	$ 342.9	$ 315.5	$ 1.1	$ 2.4	$ 9.1	$ 7.7	$ 0	$ 0	$ 10.2	$ 10.1	$ 0	$ 0	$ 0	$ 0	$ 5.9	$ 2.1	$ 5.9	$ 2.1	$ 0.3	$ 0.1	$ 0	$ 0	$ 0	$ 0	$ 0.3	$ 0.1	$ 0	$ 0	$ 0	$ 0	$ 2.7	$ 0.2	$ 2.7	$ 0.2									$ 20.6	$ 16.3	$ 29.0	$ 35.2	$ 29.9	$ 35.6	$ 79.5	$ 87.1	$ 98.6	$ 65.1	$ 126.2	$ 134.9	$ 19.5	$ 15.9	$ 244.3	$ 215.9
Investment in Exchange-Traded Funds																																																																			16.8	15.9	0	0	0	0	16.8	15.9
Liabilities
Risk Management Liabilities	229.0	226.6	132.2	113.6	64.8	81.3	32.0	31.7	229.0	226.6	0.4	0.5	0.6	0.6	0	0	1.0	1.1	0	0	0	0	0.7	0.3	0.7	0.3									0	0	0	0	1.8	2.7	1.8	2.7									9.5	14.3	20.3	22.0	24.9	25.2	54.7	61.5	122.3	98.8	43.9	58.7	4.6	3.5	170.8	161.0
Contingent consideration related to the acquisition of Compass																																											0	0	0	0	6.6	7.8	6.6	7.8
Settlement of contingent consideration																																																																											$ 4.3