GUARANTEES (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Guarantees
Total guarantees	$ 668.0
Expiration Less Than 1 Year	417.3
Expiration from 1 to 3 Years	31.4
Expiration Over 3 Years	219.3
Integrys Energy Services | Parental guarantees
Guarantees
Total guarantees	487.0
Guarantee exposure related to open transactions	229.7
Guarantees supporting commodity transactions
Guarantees
Total guarantees	597.1
Expiration Less Than 1 Year	368.8
Expiration from 1 to 3 Years	31.3
Expiration Over 3 Years	197.0
Guarantees supporting commodity transactions | Integrys Energy Services | Parental guarantees
Guarantees
Total guarantees	436.3
Guarantees supporting commodity transactions | MERC
Guarantees
Total guarantees	108.5
Guarantees supporting commodity transactions | MGU
Guarantees
Total guarantees	45.3
Guarantees supporting commodity transactions | IBS
Guarantees
Total guarantees	5.0
Guarantees supporting commodity transactions | UPPCO
Guarantees
Total guarantees	2.0
Standby letters of credit
Guarantees
Total guarantees	34.2
Expiration Less Than 1 Year	34.1
Expiration from 1 to 3 Years	0.1
Standby letters of credit | Integrys Energy Services | Parental guarantees
Guarantees
Total guarantees	32.5
Standby letters of credit | UPPCO, WPS, MGU, NSG, MERC, PGL, and Pinnacle CNG Systems
Guarantees
Total guarantees	1.7
Surety bonds
Guarantees
Total guarantees	14.4
Expiration Less Than 1 Year	14.4
Other guarantees
Guarantees
Total guarantees	22.3
Expiration Over 3 Years	22.3
Other guarantees | Integrys Energy Services | Texas Retail Marketing Business
Guarantees
Total guarantees	10.0
Environmental indemnification | Integrys Energy Services | Sale of Stoneman generation facility
Guarantees
Total guarantees	5.0
Indemnification and workers' compensation coverage
Guarantees
Total guarantees	$ 7.3