REGULATORY ENVIRONMENT (Details) (USD $) In Millions, unless otherwise specified				12 Months Ended							12 Months Ended	3 Months Ended																			0 Months Ended	1 Months Ended	16 Months Ended		1 Months Ended
	Mar. 29, 2013 Public Service Commission of Wisconsin (PSCW) 2014 Rates Entity's Original Request WPS	Mar. 29, 2013 Public Service Commission of Wisconsin (PSCW) 2014 Rates Retail Electric Rate Case Entity's Original Request WPS	Mar. 29, 2013 Public Service Commission of Wisconsin (PSCW) 2014 Rates Natural Gas Rate Case Entity's Original Request WPS	Dec. 31, 2012 Public Service Commission of Wisconsin (PSCW) 2013 Rates Settlement agreement WPS	Dec. 06, 2012 Public Service Commission of Wisconsin (PSCW) 2013 Rates Settlement agreement WPS	Mar. 31, 2013 Public Service Commission of Wisconsin (PSCW) 2013 Rates Retail Electric Rate Case Settlement agreement WPS	Dec. 06, 2012 Public Service Commission of Wisconsin (PSCW) 2013 Rates Retail Electric Rate Case Settlement agreement WPS	Dec. 06, 2012 Public Service Commission of Wisconsin (PSCW) 2013 Rates Natural Gas Rate Case Settlement agreement WPS	Dec. 09, 2011 Public Service Commission of Wisconsin (PSCW) 2012 Rates Retail Electric Rate Case Rate Order WPS	Dec. 09, 2011 Public Service Commission of Wisconsin (PSCW) 2012 Rates Natural Gas Rate Case Rate Order WPS	Dec. 31, 2012 Michigan Public Service Commission (MPSC) - UPPCO 2012 Rates item	Sep. 30, 2012 Michigan Public Service Commission (MPSC) - UPPCO 2012 Rates Retail Electric Rate Case Settlement agreement	Mar. 31, 2012 Michigan Public Service Commission (MPSC) - UPPCO 2012 Rates Retail Electric Rate Case Settlement agreement	Dec. 20, 2011 Michigan Public Service Commission (MPSC) - UPPCO 2012 Rates Retail Electric Rate Case Settlement agreement	Jul. 31, 2012 Illinois Commerce Commission (ICC) - PGL 2013 Rates Natural Gas Rate Case Entity's Original Request	Mar. 31, 2013 Illinois Commerce Commission (ICC) - PGL 2013 Rates Natural Gas Rate Case Entity's Revised Request	Apr. 26, 2013 Illinois Commerce Commission (ICC) - PGL 2013 Rates Natural Gas Rate Case Regulatory Authority Staff Recommendation	Mar. 31, 2013 Illinois Commerce Commission (ICC) - PGL 2013 Rates Natural Gas Rate Case Regulatory Authority Staff Recommendation	Mar. 31, 2013 Illinois Commerce Commission (ICC) - PGL 2013 Rates Natural Gas Rate Case Rebuttal testimony Maximum	Mar. 31, 2013 Illinois Commerce Commission (ICC) - PGL 2012 Rates	Jan. 10, 2012 Illinois Commerce Commission (ICC) - PGL 2012 Rates Natural Gas Rate Case Rate Order	Jul. 31, 2012 Illinois Commerce Commission (ICC) - NSG 2013 Rates Natural Gas Rate Case Entity's Original Request	Mar. 31, 2013 Illinois Commerce Commission (ICC) - NSG 2013 Rates Natural Gas Rate Case Entity's Revised Request	Apr. 26, 2013 Illinois Commerce Commission (ICC) - NSG 2013 Rates Natural Gas Rate Case Regulatory Authority Staff Recommendation	Mar. 31, 2013 Illinois Commerce Commission (ICC) - NSG 2013 Rates Natural Gas Rate Case Regulatory Authority Staff Recommendation	Mar. 31, 2013 Illinois Commerce Commission (ICC) - NSG 2013 Rates Natural Gas Rate Case Rebuttal testimony Maximum	Mar. 31, 2013 Illinois Commerce Commission (ICC) - NSG 2012 Rates	Jan. 10, 2012 Illinois Commerce Commission (ICC) - NSG 2012 Rates Natural Gas Rate Case Rate Order	Apr. 26, 2013 Illinois Commerce Commission (ICC) - PGL and Illinois Commerce Commission (ICC) - NSG 2013 Rates Natural Gas Rate Case Regulatory Authority Staff Recommendation	Mar. 31, 2013 Illinois Commerce Commission (ICC) - PGL and Illinois Commerce Commission (ICC) - NSG 2013 Rates Natural Gas Rate Case Regulatory Authority Staff Recommendation	Jul. 13, 2012 Minnesota Public Utilities Commission (MPUC) 2011 Rates Natural Gas Rate Case Rate Order	May 31, 2010 Federal Energy Regulatory Commission (FERC) Seams Elimination Charge Adjustment (SECA) Integrys Energy Services item	Mar. 31, 2006 Federal Energy Regulatory Commission (FERC) Seams Elimination Charge Adjustment (SECA) Integrys Energy Services	Mar. 31, 2013 Federal Energy Regulatory Commission (FERC) Seams Elimination Charge Adjustment (SECA) Integrys Energy Services	Jan. 31, 2013 Federal Energy Regulatory Commission (FERC) Settlement agreement Integrys Energy Services	Jan. 31, 2013 Michigan Public Service Commission (MPSC) - MGU
Regulatory Proceedings:
Rate increase (decrease)		$ 71.1	$ 19.0												$ 78.3	$ 97.0						$ 9.8	$ 9.6
Percent return on equity	10.75%														10.75%	10.00%						10.75%	10.00%
Equity ratio percent	51.11%														50.00%							50.00%
Rate increase (decrease)						0	28.5	(3.4)	8.1	(7.2)				4.2							57.8							1.9			11.0
Expenses related to electric decoupling													1.5
Reversal expenses related to electric decoupling												1.5
Estimated fuel refund including carrying costs							20.5																											3.8
Deferred regulatory liability related to employee benefits								2.4
Annual cap on rates							14.0	8.0
Percent return on equity					10.30%									10.20%							9.45%							9.45%			9.70%
Equity ratio percent					51.61%									54.90%							49.00%							50.00%			50.48%
Authorized recovery of income tax amounts previously expensed related to the Federal Health Care Reform Act				5.9
Percentage of price variance from the cost of fuel and purchased power included in rates									2.00%
Trial basis period																															3 years
Transition period when charges paid																																	16 months
Billings received																																	19.2
Rate increase (decrease)																	52.0	12.9	15.4					6.1	3.4	2.6
Percent return on equity																													9.33%	9.06%
Equity ratio percent																	50.43%	50.43%						50.32%	50.32%
Annual cap on distribution revenue														1.50%																	10.00%
Number of decoupling mechanisms in place											0
Favorable rulings, number of issues																																2
Modification of depreciation study as a result of reversal of rulings																																				2.5
Reduction to depreciation expense																																				2.5
Settlement agreement amount																																			9.5
Period from the effective date of agreement within which lump sum payment is to be made																																			5 days
Period from the effective date of agreement within which petitions for review filed with U.S. Court of Appeals is to be withdrawn																																			5 days
Amount expected to be recovered or refunded related to decoupling mechanisms																				$ 14.8							$ 1.7