REGULATORY ENVIRONMENT (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				3 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2013	Mar. 31, 2013	Oct. 31, 2014	Nov. 30, 2013	Jun. 30, 2013	Jul. 31, 2012	Feb. 28, 2015	Jan. 31, 2015
WPS
Public Utilities, General Disclosures [Line Items]
Percentage price variance from rate case-approved fuel and purchased power costs before deferral is required	2.00%
Public Service Commission of Wisconsin (PSCW) | 2015 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved return on common equity percent reflected in rates of customers	10.20%
Approved percent of capital structure composed of common equity	50.28%
Percentage price variance from rate case-approved fuel and purchased power costs before deferral is required	2.00%
Public Service Commission of Wisconsin (PSCW) | 2014 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved return on common equity percent reflected in rates of customers		10.20%
Approved percent of capital structure composed of common equity		50.14%
Public Service Commission of Wisconsin (PSCW) | 2013 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved return on common equity percent reflected in rates of customers			10.30%
Approved percent of capital structure composed of common equity			51.61%
Recovery of income tax amounts previously expensed related to the Federal Health Care Reform Act			$ 5.9
Public Service Commission of Wisconsin (PSCW) | Retail electric rates | 2015 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers	24.6
Public Service Commission of Wisconsin (PSCW) | Retail electric rates | 2014 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers		(12.8)
Public Service Commission of Wisconsin (PSCW) | Retail electric rates | 2013 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers			28.5
Fuel refund			20.5
Deferral related to pension and other employee benefit costs			7.3
Annual cap for decoupling			14.0
Public Service Commission of Wisconsin (PSCW) | Retail natural gas rates | 2015 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers	(15.4)
Public Service Commission of Wisconsin (PSCW) | Retail natural gas rates | 2014 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers		4.0
Public Service Commission of Wisconsin (PSCW) | Retail natural gas rates | 2013 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers			(3.4)
Deferral related to pension and other employee benefit costs			2.1
Annual cap for decoupling			8.0
Michigan Public Service Commission (MPSC) | MGU
Public Utilities, General Disclosures [Line Items]
Reversal of disallowance of depreciation expense associated with the retirement of certain assets				2.5
Reduction to depreciation expense					2.5
Michigan Public Service Commission (MPSC) | Retail electric rates | 2015 Rates | WPS
Public Utilities, General Disclosures [Line Items]
Requested annual increase (decrease) in rates for customers						5.7
Requested return on common equity percent						10.60%
Requested percent of capital structure composed of common equity						50.48%
Michigan Public Service Commission (MPSC) | Retail electric rates | 2014 Rates | UPPCO
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers		5.8
Approved return on common equity percent reflected in rates of customers		10.15%
Approved percent of capital structure composed of common equity		56.74%
Michigan Public Service Commission (MPSC) | Retail natural gas rates | 2014 Rates | MGU
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers							4.5
Approved return on common equity percent reflected in rates of customers							10.25%
Approved percent of capital structure composed of common equity							48.62%
Illinois Commerce Commission (ICC) | Retail natural gas rates | 2013 Rates | NSG
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers								6.6
Approved return on common equity percent reflected in rates of customers								9.28%
Approved percent of capital structure composed of common equity								50.32%
Adjustment to annual increase (decrease) in rates for customers		0.1
Illinois Commerce Commission (ICC) | Retail natural gas rates | 2013 Rates | PGL
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers								57.2
Approved return on common equity percent reflected in rates of customers								9.28%
Approved percent of capital structure composed of common equity								50.43%
Adjustment to annual increase (decrease) in rates for customers		2.6
Minnesota Public Utilities Commission (MPUC) | Retail natural gas rates | 2014 Rates | MERC
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers						7.6
Approved return on common equity percent reflected in rates of customers						9.35%
Approved percent of capital structure composed of common equity						50.31%
Interim rates to be refunded to customers	3.1
Annual percent cap on decoupling based on distribution revenues approved in the rate case						10.00%
Minnesota Public Utilities Commission (MPUC) | Retail natural gas rates | 2011 Rates | MERC
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers									11.0
Approved return on common equity percent reflected in rates of customers									9.70%
Approved percent of capital structure composed of common equity									50.48%
Trial basis period for decoupling mechanism									3 years
Annual percent cap on decoupling based on distribution revenues approved in the rate case									10.00%
Subsequent Event | Illinois Commerce Commission (ICC) | Retail natural gas rates | 2015 Rates | NSG
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers											3.7
Approved return on common equity percent reflected in rates of customers											9.05%
Approved percent of capital structure composed of common equity											50.48%
Amended approved annual increase (decrease) in rates for customers										3.5
Subsequent Event | Illinois Commerce Commission (ICC) | Retail natural gas rates | 2015 Rates | PGL
Public Utilities, General Disclosures [Line Items]
Approved annual increase (decrease) in rates for customers											74.8
Approved return on common equity percent reflected in rates of customers											9.05%
Approved percent of capital structure composed of common equity											50.33%
Amended approved annual increase (decrease) in rates for customers										$ 71.1