GUARANTEES (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Guarantees
Total guarantees	$ 690.8
Guarantees expiring in less than 1 year	446.6
Guarantees expiring within 1 to 3 years	7.8
Guarantees with expiration over 3 years from balance sheet date	236.4
Integrys Energy Services | Parental guarantees
Guarantees
Total guarantees	496.0
Guarantee exposure related to open transactions	245.8
Guarantees supporting commodity transactions
Guarantees
Total guarantees	605.9
Guarantees expiring in less than 1 year	390.3
Guarantees expiring within 1 to 3 years	2.7
Guarantees with expiration over 3 years from balance sheet date	212.9
Guarantees supporting commodity transactions | Integrys Energy Services
Guarantees
Total guarantees	438.3
Guarantees supporting commodity transactions | MERC
Guarantees
Total guarantees	110.5
Guarantees supporting commodity transactions | IBS
Guarantees
Total guarantees	5.0
Guarantees supporting commodity transactions | MGU
Guarantees
Total guarantees	50.1
Guarantees supporting commodity transactions | UPPCO
Guarantees
Total guarantees	2.0
Standby letters of credit
Guarantees
Total guarantees	40.8
Guarantees expiring in less than 1 year	40.7
Guarantees expiring within 1 to 3 years	0.1
Guarantees with expiration over 3 years from balance sheet date	0
Standby letters of credit | Integrys Energy Services
Guarantees
Total guarantees	38.8
Standby letters of credit | ITF, MERC, MGU, NSG, PGL, UPPCO, and WPS
Guarantees
Total guarantees	2.0
Surety bonds
Guarantees
Total guarantees	20.6
Guarantees expiring in less than 1 year	15.6
Guarantees expiring within 1 to 3 years	5.0
Guarantees with expiration over 3 years from balance sheet date	0
Other guarantees
Guarantees
Total guarantees	23.5
Guarantees expiring in less than 1 year	0
Guarantees expiring within 1 to 3 years	0
Guarantees with expiration over 3 years from balance sheet date	23.5
Other guarantees | Integrys Energy Services | Texas Retail Electric Marketing Business
Guarantees
Total guarantees	10.0
Other guarantees | ITF
Guarantees
Total guarantees	2.4
Environment indemnification other guarantee | Integrys Energy Services | Sale of Stoneman generation facility
Guarantees
Total guarantees	5.0
Indemnification and workers' compensation coverage
Guarantees
Total guarantees	$ 6.1