GUARANTEES (Details) (USD $) In Millions, unless otherwise specified		1 Months Ended														12 Months Ended
	Dec. 31, 2011	Mar. 31, 2010 Integrys Energy Services United States Wholesale Electric Marketing and Trading Business	Dec. 31, 2011 Integrys Energy Services Parental guarantees	Dec. 31, 2011 Guarantees supporting commodity transactions	Dec. 31, 2011 Guarantees supporting commodity transactions Integrys Energy Services Parental guarantees	Dec. 31, 2011 Guarantees supporting commodity transactions MERC	Dec. 31, 2011 Guarantees supporting commodity transactions MGU	Dec. 31, 2011 Guarantees supporting commodity transactions IBS	Dec. 31, 2011 Guarantees supporting commodity transactions UPPCO	Dec. 31, 2011 Standby letters of credit	Dec. 31, 2011 Standby letters of credit Integrys Energy Services Parental guarantees	Dec. 31, 2011 Standby letters of credit UPPCO, WPS, MGU, NSG, MERC, and PGL	Dec. 31, 2011 Surety bonds	Dec. 31, 2011 Surety bonds Integrys Energy Services Parental guarantees	Dec. 31, 2011 Other guarantees	Dec. 31, 2011 Other guarantees Integrys Energy Services United States Wholesale Electric Marketing and Trading Business y	Dec. 31, 2011 Other guarantees Integrys Energy Services Texas Retail Electric Marketing Business	Dec. 31, 2011 Other guarantees Integrys Energy Services Parental guarantees	Dec. 31, 2011 Environmental indemnification Integrys Energy Services Sale of Stoneman generation facility	Dec. 31, 2011 Indemnification and workers' compensation coverage Integrys Energy Services
Guarantees
Total guarantees	$ 734.5		$ 532.0	$ 614.2	$ 431.8	$ 119.1	$ 56.3	$ 5.0	$ 2.0	$ 64.0	$ 61.5	$ 2.5	$ 13.5	$ 3.2	$ 42.8	$ 20.0	$ 10.0	$ 35.5	$ 5.0	$ 7.8
Guarantees expiring in less than 1 year	434.5			387.6						33.4			13.5
Guarantees expiring within 1 to 3 years	55.5			4.9						30.6					20.0
Guarantees with expiration over 3 years from balance sheet date	244.5			221.7											22.8
Period of retention of counter party payment default risk (in years)																2
Percentage of counterparties transferred in the transaction		50.00%														50.00%
Guarantee exposure related to open transactions			$ 311.1