RESTRUCTURING EXPENSE (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Restructuring costs
Total restructuring expense	$ 2.0	$ 7.9	$ 43.5
Integrys Energy Services | Strategy Change
Restructuring costs
Employee-related costs	(0.1)	1.1	10.1
Professional fees		6.4	9.2
Software write-offs			5.3
Accelerated lease costs and depreciation	1.9	0.4	0.6
Miscellaneous		0.4	0.3
Total restructuring expense	1.8	8.3	25.5
Reductions in Workforce
Activity related to restructuring costs
Accrued restructuring costs at beginning of period	0.2	18.0
Add: Adjustments to accrual during the period		(0.1)
Deduct: Cash payments	0.2	17.7
Accrued restructuring costs at end of period		0.2	18.0
Restructuring expense billed	(0.2)	0.3
Restructuring costs
Total restructuring expense	0.2	(0.4)	18.0
Reductions in Workforce | Electric Utility
Restructuring costs
Total restructuring expense	0.2	(0.3)	8.6
Reductions in Workforce | Natural Gas Utility
Restructuring costs
Total restructuring expense		(0.2)	6.9
Reductions in Workforce | Integrys Energy Services
Restructuring costs
Total restructuring expense			1.7
Reductions in Workforce | Holding Company and Other
Restructuring costs
Total restructuring expense		0.1	0.8
Employee-related restructuring expense | Integrys Energy Services | Strategy Change
Activity related to restructuring costs
Accrued restructuring costs at beginning of period	0.3	8.2
Add: Adjustments to accrual during the period	(0.1)	1.1
Deduct: Cash payments	0.2	9.0
Accrued restructuring costs at end of period		$ 0.3