Business Segments (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($) a	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) a Segments	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments			2
Operating Revenue	$ 345.4	$ 293.9	$ 1,020.2	$ 848.7
Net Income (Loss) Attributable to ALLETE	27.6	40.7	107.3	127.1
Assets	6,332.2		$ 6,332.2		$ 6,084.6
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments			3
Operating Revenue	304.8	255.1	$ 888.2	721.2
Net Income (Loss) Attributable to ALLETE	32.9	42.4	99.4	111.0
Assets	4,207.4		4,207.4		4,196.8
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue	37.0	34.2	118.0	102.5
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue	44.3	37.1	124.7	103.2
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue	14.4	11.2	38.5	30.5
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue	139.7	109.6	407.8	316.4
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue	41.2	30.9	116.9	96.6
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	25.8	29.7	79.9	69.6
Regulated Operations [Member] | CIP Financial Incentive [Member]
Business Segments [Line Items]
Operating Revenue	2.4	2.4	2.4	2.4
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue	16.9	14.8	59.4	52.5
Net Income (Loss) Attributable to ALLETE	(0.8)	1.1	11.7	16.8
Assets	1,691.4		1,691.4		1,483.3
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue	14.0	11.9	50.8	44.0
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue	$ 2.9	2.9	$ 8.6	8.5
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments			2
Land in Minnesota (Acres) | a	4,000		4,000
Operating Revenue	$ 23.7	24.0	$ 72.6	75.0
Net Income (Loss) Attributable to ALLETE	(4.5)	(2.8)	(3.8)	(0.7)
Assets	433.4		433.4		$ 404.5
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue	20.3	20.7	63.4	65.6
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	$ 3.4	$ 3.3	$ 9.2	$ 9.4