Business Segments (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019 USD ($) a	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) a Segments	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments				3
Operating Revenue		$ 288.3	$ 348.0	$ 935.9	$ 1,050.3
Net Income (Loss)		31.2	30.7	135.9	113.0
Gain on Sale of U.S. Water Services, After-tax				11.1
Assets		5,275.8		$ 5,275.8		$ 5,165.0
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments				3
Operating Revenue		254.1	261.3	$ 786.1	789.3
Net Income (Loss)		32.4	29.8	114.2	99.7
Assets		4,045.2		4,045.2		3,952.5
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue		29.8	30.8	104.2	102.2
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue		36.8	38.1	110.6	110.9
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue		11.8	14.2	39.4	41.9
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue		115.6	117.3	357.5	347.5
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue		37.3	40.9	111.9	127.3
Regulated Operations [Member] | CIP Financial Incentive [Member]
Business Segments [Line Items]
Operating Revenue		2.8	3.0	2.8	3.0
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		20.0	17.0	59.7	56.5
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		10.4	15.0	43.3	57.9
Net Income (Loss)		(1.2)	1.0	6.5	15.9
Assets		839.8		839.8		606.6
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue		7.5	9.0	34.6	40.0
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue		2.9	6.0	8.7	17.9
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	45.0	33.4	124.7
Net Income (Loss)	[1]	0.0	1.7	(1.1)	0.5
Gain on Sale of U.S. Water Services, After-tax				11.1
Assets	[2]	0.0		0.0		295.8
U.S. Water Services [Member] | Point-in-Time [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	29.0	19.0	77.0
U.S. Water Services [Member] | Contract [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	9.7	9.2	28.7
U.S. Water Services [Member] | Capital Project [Member]
Business Segments [Line Items]
Operating Revenue	[1]	$ 0.0	6.3	$ 5.2	19.0
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		$ 23.8	26.7	$ 73.1	78.4
Net Income (Loss)	[1]	0.0	(1.8)	16.3	(3.1)
Assets		390.8		390.8		$ 310.1
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue		20.5	21.2	61.9	63.9
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		$ 3.3	$ 5.5	$ 11.2	$ 14.5

[1] On March 26, 2019, ALLETE completed the sale of U.S. Water Services. The Company recognized a gain on the sale of $11.1 million after-tax reflected in Corporate and Other in 2019. (See Note 1. Operations and Significant Accounting Policies.)

[2] On March 26, 2019, ALLETE completed the sale of U.S. Water Services. (See Note 1. Operations and Significant Accounting Policies.)