GUARANTEES (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013
Guarantor Obligations [Line Items]
Total guarantees	$ 660.6
Guarantees expiring in less than 1 year	397.4
Guarantees expiring within 1 to 3 years	29.5
Guarantees with expiration over 3 years from balance sheet date	233.7
Integrys Energy Services
Guarantor Obligations [Line Items]
Guarantee exposure related to open transactions	250.4
Integrys Energy Services | Standby letters of credit
Guarantor Obligations [Line Items]
Total guarantees	29.2
Integrys Energy Services | Guarantees supporting commodity transactions
Guarantor Obligations [Line Items]
Total guarantees	430.4
Integrys Energy Services | Parental guarantees
Guarantor Obligations [Line Items]
Total guarantees	478.5
MERC | Guarantees supporting commodity transactions
Guarantor Obligations [Line Items]
Total guarantees	109.0
IBS | Guarantees supporting commodity transactions
Guarantor Obligations [Line Items]
Total guarantees	5.0
UPPCO, WPS, MGU, NSG, MERC, PGL, and Pinnacle CNG Systems | Standby letters of credit
Guarantor Obligations [Line Items]
Total guarantees	2.0
MGU | Guarantees supporting commodity transactions
Guarantor Obligations [Line Items]
Total guarantees	40.6
UPPCO | Guarantees supporting commodity transactions
Guarantor Obligations [Line Items]
Total guarantees	2.0
Guarantees supporting commodity transactions
Guarantor Obligations [Line Items]
Total guarantees	587.0
Guarantees expiring in less than 1 year	346.3
Guarantees expiring within 1 to 3 years	28.2
Guarantees with expiration over 3 years from balance sheet date	212.5
Standby letters of credit
Guarantor Obligations [Line Items]
Total guarantees	31.2
Guarantees expiring in less than 1 year	31.0
Guarantees expiring within 1 to 3 years	0.2
Guarantees with expiration over 3 years from balance sheet date	0
Surety bonds
Guarantor Obligations [Line Items]
Total guarantees	21.2
Guarantees expiring in less than 1 year	20.1
Guarantees expiring within 1 to 3 years	1.1
Guarantees with expiration over 3 years from balance sheet date	0
Other guarantees
Guarantor Obligations [Line Items]
Total guarantees	21.2
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	21.2
Other guarantees | Integrys Energy Services | Texas Retail Electric Marketing Business
Guarantor Obligations [Line Items]
Total guarantees	10.0
Indemnification and workers' compensation coverage
Guarantor Obligations [Line Items]
Total guarantees	6.2
Environment Indemnification Guarantee Other | Integrys Energy Services | Sale of Stoneman generation facility
Guarantor Obligations [Line Items]
Total guarantees	$ 5.0