BUSINESS SEGMENTS - Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Business Segments [Line Items]
Total Operating Revenue	$ 354.5	$ 533.4	$ 757.8	$ 1,098.3
Net Income (Loss) Attributable to ALLETE	33.0	51.5	83.7	109.7
Assets	6,661.3		6,661.3		$ 6,656.4
Corporate and Other
Business Segments [Line Items]
Total Operating Revenue	354.5	533.4	757.8	1,098.3
Regulated Operations
Business Segments [Line Items]
Net Income (Loss) Attributable to ALLETE	33.7	37.8	77.9	78.4
Assets	4,404.2		4,404.2		4,335.0
Regulated Operations | Operating Segments
Business Segments [Line Items]
Total Operating Revenue	279.8	292.2	618.1	604.8
Regulated Operations | Residential | Operating Segments
Business Segments [Line Items]
Operating Revenue	37.1	36.0	88.6	85.4
Regulated Operations | Commercial | Operating Segments
Business Segments [Line Items]
Operating Revenue	43.8	44.0	93.5	91.7
Regulated Operations | Municipal | Operating Segments
Business Segments [Line Items]
Operating Revenue	7.4	7.6	16.4	16.5
Regulated Operations | Industrial | Operating Segments
Business Segments [Line Items]
Operating Revenue	146.8	140.2	306.3	285.1
Regulated Operations | Other Power Suppliers | Operating Segments
Business Segments [Line Items]
Operating Revenue	30.4	36.3	70.4	72.2
Regulated Operations | Other | Operating Segments
Business Segments [Line Items]
Operating Revenue	14.3	28.1	42.9	53.9
ALLETE Clean Energy
Business Segments [Line Items]
Net Income (Loss) Attributable to ALLETE	2.4	3.1	6.2	11.6
Assets	1,577.7		1,577.7		1,594.1
ALLETE Clean Energy | Operating Segments
Business Segments [Line Items]
Total Operating Revenue	15.7	181.1	34.8	382.6
ALLETE Clean Energy | Long-term PSA | Operating Segments
Business Segments [Line Items]
Operating Revenue	14.5	13.2	32.3	31.6
ALLETE Clean Energy | Sale of Wind Energy Facilities | Operating Segments
Business Segments [Line Items]
Operating Revenue	0.0	166.6	0.0	348.4
ALLETE Clean Energy | Other | Operating Segments
Business Segments [Line Items]
Operating revenue, other than customer revenue	1.2	1.3	2.5	2.6
Corporate and Other
Business Segments [Line Items]
Net Income (Loss) Attributable to ALLETE	(3.1)	10.6	(0.4)	19.7
Assets	679.4		679.4		$ 727.3
Corporate and Other | Corporate and Other
Business Segments [Line Items]
Total Operating Revenue	59.0	60.1	104.9	110.9
Corporate and Other | Long-term Contract | Corporate and Other
Business Segments [Line Items]
Operating Revenue	27.3	23.9	52.7	49.4
Corporate and Other | Sale of Renewable Development Projects | Corporate and Other
Business Segments [Line Items]
Operating Revenue	25.6	32.6	39.5	52.4
Corporate and Other | Other | Corporate and Other
Business Segments [Line Items]
Operating Revenue	$ 6.1	$ 3.6	$ 12.7	$ 9.1