LONG-TERM DEBT (Details) (USD $) In Millions, unless otherwise specified																									1 Months Ended						1 Months Ended																			1 Months Ended		1 Months Ended																			1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Other term loan	Dec. 31, 2010 Other term loan	Apr. 30, 2001 Other term loan	Dec. 31, 2011 WPS First Mortgage Bonds	Dec. 31, 2010 WPS First Mortgage Bonds	Dec. 31, 2011 WPS 6.125% Senior Notes	Aug. 31, 2011 WPS 6.125% Senior Notes	Dec. 31, 2010 WPS 6.125% Senior Notes	Dec. 31, 2011 WPS 4.875% Senior Notes due 2012	Dec. 31, 2010 WPS 4.875% Senior Notes due 2012	Dec. 31, 2011 WPS 4.80% Senior Notes due 2013	Dec. 31, 2010 WPS 4.80% Senior Notes due 2013	Dec. 31, 2011 WPS 3.95% Senior Notes due 2013	Dec. 31, 2010 WPS 3.95% Senior Notes due 2013	Dec. 31, 2011 WPS 6.375% Senior Notes due 2015	Dec. 31, 2010 WPS 6.375% Senior Notes due 2015	Dec. 31, 2011 WPS 5.65% Senior Notes due 2017	Dec. 31, 2010 WPS 5.65% Senior Notes due 2017	Dec. 31, 2011 WPS 6.08% Senior Notes due 2028	Dec. 31, 2010 WPS 6.08% Senior Notes due 2028	Dec. 31, 2011 WPS 5.55% Senior Notes due 2036	Dec. 31, 2010 WPS 5.55% Senior Notes due 2036	Nov. 30, 2011 UPPCO First Mortgage Bonds	Dec. 31, 2011 UPPCO First Mortgage Bonds	Dec. 31, 2010 UPPCO First Mortgage Bonds	Dec. 31, 2011 PELLC Unsecured 6.9% senior notes	Jan. 31, 2011 PELLC Unsecured 6.9% senior notes	Dec. 31, 2010 PELLC Unsecured 6.9% senior notes	Nov. 30, 2011 PGL	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series KK, 5.00% bonds due 2033	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series KK, 5.00% bonds due 2033	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series NN-2, 4.625% bonds due 2013	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series NN-2, 4.625% bonds due 2013	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series QQ, 4.875% bonds due 2038	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series QQ, 4.875% bonds due 2038	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series RR, 4.30% bonds due 2035	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series RR, 4.30% bonds due 2035	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series SS, 7.00% bonds due 2013	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series SS, 7.00% bonds due 2013	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series TT, 8.00% bonds due 2018	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series TT, 8.00% bonds due 2018	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series UU, 4.63% bonds due 2019	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series UU, 4.63% bonds due 2019	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series VV, 2.125% bonds due 2030	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series VV, 2.125% bonds due 2030	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series WW, 2.625% bonds due 2033	Dec. 31, 2010 PGL Fixed First and Refunding Mortgage Bonds, Series WW, 2.625% bonds due 2033	Nov. 30, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series XX, 2.21% bonds due 2016	Dec. 31, 2011 PGL Fixed First and Refunding Mortgage Bonds, Series XX, 2.21% bonds due 2016	Aug. 31, 2011 PGL Series OO bonds	Dec. 31, 2010 PGL Series OO bonds	Dec. 31, 2011 NSG First Mortgage Series M, 5.00% bonds due 2028	Dec. 31, 2010 NSG First Mortgage Series M, 5.00% bonds due 2028	Dec. 31, 2011 NSG First Mortgage Series N-2, 4.625% bonds due 2013	Dec. 31, 2010 NSG First Mortgage Series N-2, 4.625% bonds due 2013	Dec. 31, 2011 NSG First Mortgage Series O, 7.00% bonds due 2013	Dec. 31, 2010 NSG First Mortgage Series O, 7.00% bonds due 2013	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC.	Dec. 31, 2010 INTEGRYS ENERGY GROUP, INC.	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC. Maximum	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC. 5.375% Unsecured Senior Notes due 2012	Dec. 31, 2010 INTEGRYS ENERGY GROUP, INC. 5.375% Unsecured Senior Notes due 2012	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC. 7.27% Unsecured Senior Notes due 2014	Dec. 31, 2010 INTEGRYS ENERGY GROUP, INC. 7.27% Unsecured Senior Notes due 2014	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC. 8.00% Unsecured Senior Notes due 2016	Dec. 31, 2010 INTEGRYS ENERGY GROUP, INC. 8.00% Unsecured Senior Notes due 2016	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC. 4.17% Unsecured Senior Notes due 2020	Dec. 31, 2010 INTEGRYS ENERGY GROUP, INC. 4.17% Unsecured Senior Notes due 2020	May 31, 2011 INTEGRYS ENERGY GROUP, INC. Junior Subordinated Notes	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC. Junior Subordinated Notes	Dec. 31, 2010 INTEGRYS ENERGY GROUP, INC. Junior Subordinated Notes	Dec. 31, 2011 INTEGRYS ENERGY GROUP, INC. Junior Subordinated Notes Maximum
LONG-TERM DEBT
Interest rate stated percentage						7.125%		6.125%			4.875%		4.80%		3.95%		6.375%		5.65%		6.08%		5.55%			9.32%		6.90%				5.00%		4.625%		4.875%		4.30%		7.00%		8.00%		4.63%		2.125%		2.625%			2.21%			5.00%		4.625%		7.00%					5.375%		7.27%		8.00%		4.17%			6.11%
Fair value hedge adjustment																														$ 0.9
Total	2,123.6	2,640.2	27.0	27.0	27.0	0.1	0.1		150.0	150.0	150.0	150.0	125.0	125.0	22.0	22.0	125.0	125.0	125.0	125.0	50.0	50.0	125.0	125.0			9.4			325.0		50.0	50.0	75.0	75.0	75.0	75.0	50.0	50.0	45.0	45.0	5.0	5.0	75.0	75.0	50.0	50.0	50.0	50.0		50.0		51.0	28.2	28.3	40.0	40.0	6.5	6.5				100.0	100.0	100.0	100.0	55.0	55.0	250.0	250.0		269.8	300.0
Unamortized discount and premium on bonds and debt	(1.6)	(1.7)
Total debt	2,122.0	2,638.5
Less current portion	(250.0)	(476.9)																																																										(100.0)
Total long-term debt	1,872.0	2,161.6									150.0																																																	674.6	804.7		100.0
Settlement of derivative instrument related to senior notes																													50.0
Amount of debt issued																															50.0																			50.0
Amount of notes bought back																									$ 9.4																											$ 51.0																			$ 30.2
Percentage of notes that may not be bought back in excess of principal amount outstanding, under the replacement capital covenant (as a percent)																																																																										10.00%
Interest rate as of reporting date (as a percent)			0.09%
Maximum debt to total capitalization ratio (as a percent)																																																														65.00%