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				1 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 WPS	Dec. 31, 2012 WPS	Sep. 30, 2013 WPS Senior Notes, 4.80% due 2013	Feb. 28, 2013 WPS Senior Notes, 3.95% due 2013	Sep. 30, 2013 PGL	Dec. 31, 2012 PGL	Apr. 30, 2013 PGL Fixed First and Refunding Mortgage Bonds, Series KK, 5.00% bonds due 2033	May 31, 2013 PGL Fixed First and Refunding Mortgage Bonds, Series NN-2, 4.625% bonds due 2013	Sep. 30, 2013 PGL Fixed First and Refunding Mortgage Bonds, Series SS, 7.00% bonds due 2013	Apr. 30, 2013 PGL Fixed First and Refunding Mortgage Bonds, Series ZZ, 4.0% bonds due 2033	Aug. 31, 2013 PGL Fixed First and Refunding Mortgage Bonds, Series AAA, 3.96% bonds due 2043	Sep. 30, 2013 NSG	Dec. 31, 2012 NSG	May 31, 2013 NSG First Mortgage Series N-2, 4.625% bonds due 2013	Sep. 30, 2013 NSG First Mortgage Series O, 7.00% bonds due 2013	May 31, 2013 NSG First Mortgage Series Q, 3.96% bonds due 2043	Sep. 30, 2013 Integrys Energy Group, Inc.	Dec. 31, 2012 Integrys Energy Group, Inc.	Sep. 30, 2013 Integrys Energy Group, Inc. Unsecured Senior Notes, 7.27% due 2014	Aug. 31, 2013 Integrys Energy Group, Inc. Junior Subordinated Notes, 6.00% due 2073
LONG-TERM DEBT
Total	$ 2,783.4	$ 2,246.4	$ 850.1	$ 872.1			$ 770.0	$ 625.0						$ 88.5	$ 74.5				$ 1,074.8	$ 674.8
Unamortized discount on debt	(0.7)	(1.2)
Total debt	2,782.7	2,245.2
Less current portion	276.5	313.5			125.0						45.0						6.5				100.0
Total long-term debt	2,506.2	1,931.7
Interest rate stated percentage					4.80%	3.95%			5.00%	4.625%	7.00%	4.00%	3.96%			4.625%	7.00%	3.96%			7.27%	6.00%
Principal balance repaid						22.0			50.0	75.0						40.0
Amount of debt issued												$ 50.0	$ 220.0					$ 54.0				$ 400.0