BUSINESS SEGMENTS - Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Business Segments [Line Items]
Other	$ 1.4	$ 1.2	$ 2.9	$ 2.5
Total Operating Revenue	360.3	354.5	760.5	757.8
Net Income (Loss) Attributable to ALLETE	31.9	33.0	88.0	83.7
Assets	6,913.2	6,661.3	6,913.2	6,661.3	$ 6,754.3
Operating Segments
Business Segments [Line Items]
Other	1.4	1.2	2.9	2.5
Total Operating Revenue	335.0	323.9	705.7	698.3
Net Income (Loss) Attributable to ALLETE	28.6	43.8	83.6	95.8
Assets	6,467.2	6,288.1	6,467.2	6,288.1
Corporate and Other
Business Segments [Line Items]
Other	0.0	0.0	0.0	0.0
Total Operating Revenue	25.3	30.6	54.8	59.5
Net Income (Loss) Attributable to ALLETE	3.3	(10.8)	4.4	(12.1)
Assets	446.0	373.2	446.0	373.2
Regulated Operations
Business Segments [Line Items]
Total Operating Revenue	305.1	279.8	637.9	618.1
Net Income (Loss) Attributable to ALLETE	23.0	33.7	61.4	77.9
Regulated Operations | Operating Segments
Business Segments [Line Items]
Other	0.0	0.0	0.0	0.0
Total Operating Revenue	305.1	279.8	637.9	618.1
Net Income (Loss) Attributable to ALLETE	23.0	33.7	61.4	77.9
Assets	4,633.4	4,404.2	4,633.4	4,404.2
Regulated Operations | Residential
Business Segments [Line Items]
Operating Revenue	40.0	37.1	96.4	88.6
Regulated Operations | Commercial
Business Segments [Line Items]
Operating Revenue	45.3	43.8	97.0	93.5
Regulated Operations | Municipal
Business Segments [Line Items]
Operating Revenue	8.2	7.4	17.8	16.4
Regulated Operations | Industrial
Business Segments [Line Items]
Operating Revenue	137.2	146.8	277.1	306.3
Regulated Operations | Other Power Suppliers
Business Segments [Line Items]
Operating Revenue	47.3	30.4	94.4	70.4
Regulated Operations | Other
Business Segments [Line Items]
Operating Revenue	27.1	14.3	55.2	42.9
ALLETE Clean Energy
Business Segments [Line Items]
Total Operating Revenue	15.3	15.7	31.8	34.8
Net Income (Loss) Attributable to ALLETE	0.9	2.4	8.3	6.2
ALLETE Clean Energy | Operating Segments
Business Segments [Line Items]
Other	1.4	1.2	2.9	2.5
Total Operating Revenue	15.3	15.7	31.8	34.8
Net Income (Loss) Attributable to ALLETE	0.9	2.4	8.3	6.2
Assets	1,456.8	1,577.7	1,456.8	1,577.7
ALLETE Clean Energy | Long-term PSA
Business Segments [Line Items]
Operating Revenue	13.9	14.5	28.9	32.3
ALLETE Clean Energy | Other
Business Segments [Line Items]
Other	1.4	1.2	2.9	2.5
New Energy
Business Segments [Line Items]
Total Operating Revenue	14.6	28.4	36.0	45.4
Net Income (Loss) Attributable to ALLETE	4.7	7.7	13.9	11.7
New Energy | Operating Segments
Business Segments [Line Items]
Other	0.0	0.0	0.0	0.0
Total Operating Revenue	14.6	28.4	36.0	45.4
Net Income (Loss) Attributable to ALLETE	4.7	7.7	13.9	11.7
Assets	377.0	306.2	377.0	306.2
New Energy | Sale of Renewable Development Projects
Business Segments [Line Items]
Operating Revenue	10.0	25.6	28.2	39.5
New Energy | Other
Business Segments [Line Items]
Operating Revenue	4.6	2.8	7.8	5.9
Corporate and Other
Business Segments [Line Items]
Total Operating Revenue	25.3	30.6	54.8	59.5
Net Income (Loss) Attributable to ALLETE	3.3	(10.8)	4.4	(12.1)
Corporate and Other | Long-term Contract
Business Segments [Line Items]
Operating Revenue	22.4	27.3	48.5	52.7
Corporate and Other | Other
Business Segments [Line Items]
Operating Revenue	$ 2.9	$ 3.3	$ 6.3	$ 6.8