REGULATORY ENVIRONMENT (Details) $ in Millions	1 Months Ended											6 Months Ended
	Jun. 30, 2015 USD ($)	May. 31, 2015 USD ($)	Apr. 30, 2015 USD ($)	Mar. 31, 2015	Feb. 28, 2015 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Oct. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Nov. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Jun. 30, 2015 USD ($)
Public Service Commission of Wisconsin (PSCW) | 2016 rates | WPS
Regulatory environment
Requested return on common equity (as a percent)			10.20%
Requested percent of capital structure composed of common equity			50.52%
Public Service Commission of Wisconsin (PSCW) | 2016 rates | WPS | Retail electric rates
Regulatory environment
Requested annual increase (decrease) in rates			$ 94.1
Revised requested annual increase (decrease) in rates		$ 96.9
Public Service Commission of Wisconsin (PSCW) | 2016 rates | WPS | Retail natural gas rates
Regulatory environment
Requested annual increase (decrease) in rates			9.4
Revised requested annual increase (decrease) in rates		$ 9.1
Public Service Commission of Wisconsin (PSCW) | 2015 rates | WPS
Regulatory environment
Approved return on common equity (as a percent)							10.20%
Approved percent of capital structure composed of common equity							50.28%
Public Service Commission of Wisconsin (PSCW) | 2015 rates | WPS | Retail electric rates
Regulatory environment
Approved annual increase (decrease) in rates							$ 24.6
Costs of fuel for electric generation							42.0
Year-over-year positive (negative) change in decoupling refunded to or collected from customers in rates							9.0
Customer recoveries (refunds) related to decoupling							$ (4.0)
Percent fuel costs can vary from the rate case-approved costs before deferral is required							2.00%
Public Service Commission of Wisconsin (PSCW) | 2015 rates | WPS | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates							$ (15.4)
Year-over-year positive (negative) change in decoupling refunded to or collected from customers in rates							(16.0)
Customer recoveries (refunds) related to decoupling							(8.0)
Public Service Commission of Wisconsin (PSCW) | 2014 rates | WPS
Regulatory environment
Approved return on common equity (as a percent)									10.20%
Approved percent of capital structure composed of common equity									50.14%
Public Service Commission of Wisconsin (PSCW) | 2014 rates | WPS | Retail electric rates
Regulatory environment
Approved annual increase (decrease) in rates									$ (12.8)
Customer recoveries (refunds) related to decoupling							(13.0)
Public Service Commission of Wisconsin (PSCW) | 2014 rates | WPS | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates									4.0
Customer recoveries (refunds) related to decoupling							$ 8.0		8.0
Michigan Public Service Commission (MPSC) | 2016 rates | MGU | Retail natural gas rates
Regulatory environment
Requested annual increase (decrease) in rates	$ 6.7
Requested return on common equity (as a percent)	10.50%
Requested percent of capital structure composed of common equity	50.40%
Michigan Public Service Commission (MPSC) | 2015 rates | WPS | Retail electric rates
Regulatory environment
Approved annual increase (decrease) in rates			$ 4.0
Approved return on common equity (as a percent)			10.20%
Approved percent of capital structure composed of common equity			50.48%
Period of rate implementation			3 years
Michigan Public Service Commission (MPSC) | 2014 rates | MGU | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates										$ 4.5
Approved return on common equity (as a percent)										10.25%
Approved percent of capital structure composed of common equity										48.62%
Illinois Commerce Commission (ICC) | 2015 rates | PGL | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates						$ 74.8
Approved return on common equity (as a percent)						9.05%
Approved percent of capital structure composed of common equity						50.33%
Amended approved annual increase (decrease) in rates					$ 71.1
Illinois Commerce Commission (ICC) | 2015 rates | NSG | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates						$ 3.7
Approved return on common equity (as a percent)						9.05%
Approved percent of capital structure composed of common equity						50.48%
Amended approved annual increase (decrease) in rates					$ 3.5
Illinois Commerce Commission (ICC) | 2015 rates | PGL and NSG | Retail natural gas rates
Regulatory environment
Number of appeals filed related to rehearing requests				0
Illinois Commerce Commission (ICC) | 2013 rates | PGL | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates											$ 57.2
Increase in approved customer rates									2.6
Illinois Commerce Commission (ICC) | 2013 rates | NSG | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates											$ 6.6
Increase in approved customer rates									$ 0.1
Minnesota Public Utilities Commission (MPUC) | 2014 rates | MERC | Retail natural gas rates
Regulatory environment
Approved annual increase (decrease) in rates								$ 7.6
Approved return on common equity (as a percent)								9.35%
Approved percent of capital structure composed of common equity								50.31%
Annual percent cap on decoupling based on distribution revenues approved in rate case								10.00%
Interim rates to be refunded to customers												$ 4.7
Interim rates refunded to customers												$ 3.5