SEGMENT INFORMATION (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)
Segment information
Number of reportable segments \| segment	6
WEC's ownership interest in Bishop Hill III Wind Energy Center	80.00%
Operating revenues	$ 2,377.4	$ 2,286.5
Other operation and maintenance	550.6	511.9
Depreciation and amortization	226.4	208.6
Operating income (loss)	542.8	545.1
Equity in earnings of transmission affiliates	36.1	32.8
Interest expense	124.4	106.7
Wisconsin
Segment information
Operating revenues	1,633.4	1,589.1
Illinois
Segment information
Operating revenues	536.5	507.3
Other States
Segment information
Operating revenues	185.2	169.9
Electric Transmission
Segment information
Other operation and maintenance	0.0	0.0
Depreciation and amortization	0.0	0.0
Operating income (loss)	0.0	0.0
Equity in earnings of transmission affiliates	36.1	32.8
Interest expense	$ 2.6	0.0
Non-Utility Energy Infrastructure
Segment information
Natural gas storage needs provided to Wisconsin utilities	33.00%
Operating revenues	$ 127.8	118.1
Other operation and maintenance	3.8	1.7
Depreciation and amortization	22.6	18.3
Operating income (loss)	92.7	93.0
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	15.7	16.1
Corporate and Other
Segment information
Operating revenues	1.7	1.4
Other operation and maintenance	(1.0)	(0.3)
Depreciation and amortization	6.4	7.7
Operating income (loss)	(3.9)	(5.4)
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	35.1	28.0
Reconciling Eliminations
Segment information
Other operation and maintenance	(0.7)	(96.8)
Depreciation and amortization	(4.6)	0.0
Operating income (loss)	(87.2)	0.0
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	$ (89.5)	(1.2)
ATC \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	60.00%
ATC Holdco
Segment information
Equity method investment, ownership interest (as a percent)	75.00%
Equity in earnings of transmission affiliates	$ (0.4)	(0.6)
ATC Holdco \| Electric Transmission
Segment information
Equity method investment, ownership interest (as a percent)	75.00%
Bishop Hill III Wind Energy Center \| Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Bishop Hill III Wind Energy Center	90.00%
Coyote Ridge Wind \| Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Coyote Ridge Wind, LLC	80.00%
Upstream
Segment information
WEC's ownership interest in Upstream Wind Energy Center	80.00%
Upstream \| Non-Utility Energy Infrastructure
Segment information
WEC's ownership interest in Upstream Wind Energy Center	80.00%
Total utility revenues
Segment information
Other operation and maintenance	$ 548.5	607.3
Depreciation and amortization	202.0	182.6
Operating income (loss)	541.2	457.5
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	160.5	63.8
Total utility revenues \| Wisconsin
Segment information
Other operation and maintenance	392.7	468.5
Depreciation and amortization	151.0	135.1
Operating income (loss)	361.8	273.7
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	143.4	49.4
Total utility revenues \| Illinois
Segment information
Other operation and maintenance	128.2	112.2
Depreciation and amortization	44.5	40.9
Operating income (loss)	137.9	147.6
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	14.8	12.3
Total utility revenues \| Other States
Segment information
Other operation and maintenance	27.6	26.6
Depreciation and amortization	6.5	6.6
Operating income (loss)	41.5	36.2
Equity in earnings of transmission affiliates	0.0	0.0
Interest expense	2.3	2.1
External Revenues
Segment information
Operating revenues	2,377.4	2,286.5
External Revenues \| Electric Transmission
Segment information
Operating revenues	0.0	0.0
External Revenues \| Non-Utility Energy Infrastructure
Segment information
Operating revenues	20.6	18.8
External Revenues \| Corporate and Other
Segment information
Operating revenues	1.7	1.4
External Revenues \| Reconciling Eliminations
Segment information
Operating revenues	0.0	0.0
External Revenues \| Total utility revenues
Segment information
Operating revenues	2,355.1	2,266.3
External Revenues \| Total utility revenues \| Wisconsin
Segment information
Operating revenues	1,633.4	1,589.1
External Revenues \| Total utility revenues \| Illinois
Segment information
Operating revenues	536.5	507.3
External Revenues \| Total utility revenues \| Other States
Segment information
Operating revenues	185.2	169.9
Intersegment Transactions [Member]
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Electric Transmission
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Non-Utility Energy Infrastructure
Segment information
Operating revenues	107.2	99.3
Intersegment Transactions [Member] \| Corporate and Other
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Reconciling Eliminations
Segment information
Operating revenues	(107.2)	(99.3)
Intersegment Transactions [Member] \| Total utility revenues
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Wisconsin
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Illinois
Segment information
Operating revenues	0.0	0.0
Intersegment Transactions [Member] \| Total utility revenues \| Other States
Segment information
Operating revenues	$ 0.0	$ 0.0

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