SEGMENTS INFORMATION (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment reporting information
Number of reportable segments | segment									2
Revenues	$ 361.0	$ 381.0	$ 331.5	$ 374.7	$ 337.2	$ 390.8	$ 330.3	$ 425.0	$ 1,448.2	$ 1,483.3	$ 1,683.6
Other operation and maintenance									493.2	493.4	499.7
Depreciation and amortization									124.1	121.0	116.8
Operating income (loss)	37.9	$ 105.3	$ 43.4	$ 76.9	25.5	$ 88.5	$ 44.2	$ 69.9	263.5	228.1	257.6
Other income, net									30.8	25.6	25.2
Interest expense									48.1	53.5	57.4
Capital expenditures									311.1	371.0	322.0
Total assets	4,808.2				4,505.1				4,808.2	4,505.1	4,269.3
Intersegment revenues
Segment reporting information
Revenues									0.0	0.0	0.0
Utility
Segment reporting information
Revenues									1,448.2	1,483.3	1,683.6
Other operation and maintenance									492.5	493.9	499.3
Depreciation and amortization									124.0	120.7	116.7
Operating income (loss)									264.4	228.0	257.2
Other income, net									23.3	16.0	12.3
Interest expense									48.0	53.2	55.3
Capital expenditures									311.1	371.0	322.0
Total assets	4,686.4				4,416.8				4,686.4	4,416.8	4,183.9
Utility | Intersegment revenues
Segment reporting information
Revenues									0.0	0.0	0.0
Other
Segment reporting information
Revenues									0.0	0.0	0.0
Other operation and maintenance									1.0	0.3	0.4
Depreciation and amortization									0.1	0.3	0.6
Operating income (loss)									(0.9)	0.1	0.4
Other income, net									7.5	9.6	12.9
Interest expense									0.1	0.3	2.1
Capital expenditures									0.0	0.0	0.0
Total assets	121.8				88.3				121.8	88.3	85.4
Other | Intersegment revenues
Segment reporting information
Revenues									0.3	0.8	1.4
Reconciling Eliminations
Segment reporting information
Revenues									0.0	0.0	0.0
Other operation and maintenance									(0.3)	(0.8)	0.0
Depreciation and amortization									0.0	0.0	(0.5)
Operating income (loss)									0.0	0.0	0.0
Other income, net									0.0	0.0	0.0
Interest expense									0.0	0.0	0.0
Capital expenditures									0.0	0.0	0.0
Total assets	$ 0.0				$ 0.0				0.0	0.0	0.0
Reconciling Eliminations | Intersegment revenues
Segment reporting information
Revenues									$ (0.3)	$ (0.8)	$ (1.4)
ATC
Segment reporting information
Equity method investment, ownership interest (as a percent)	34.00%								34.00%
WPS Investments, LLC
Segment reporting information
Equity method investment, ownership interest (as a percent)	10.37%								10.37%