Business Segments (Details) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019 USD ($) a	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) a Segments	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments				3
Operating Revenue		$ 290.4	$ 344.1	$ 647.6	$ 702.3
Net Income (Loss)		34.2	31.3	104.7	82.3
Approximate Gain on Sale of U.S. Water Services, After-tax				11.1
Assets		5,176.5		$ 5,176.5		$ 5,165.0
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments				3
Operating Revenue		249.8	257.8	$ 532.0	528.0
Net Income (Loss)		30.3	26.0	81.8	69.9
Assets		4,004.4		4,004.4		3,952.5
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue		29.2	30.7	74.4	71.4
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue		34.9	36.2	73.8	72.8
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue		12.2	13.7	27.6	27.7
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue		120.3	115.3	241.9	230.2
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue		35.2	42.7	74.6	86.4
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		18.0	19.2	39.7	39.5
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		15.4	18.3	32.9	42.9
Net Income (Loss)		1.9	6.8	7.7	14.9
Assets		755.0		755.0		606.6
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue		12.5	12.4	27.1	31.0
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue		2.9	5.9	5.8	11.9
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	41.5	33.4	79.7
Net Income (Loss)	[1]	0.0	0.2	(1.1)	(1.2)
Approximate 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	25.7	19.0	48.0
U.S. Water Services [Member] | Contract [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	9.5	9.2	19.0
U.S. Water Services [Member] | Capital Project [Member]
Business Segments [Line Items]
Operating Revenue	[1]	$ 0.0	6.3	$ 5.2	12.7
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		$ 25.2	26.5	$ 49.3	51.7
Net Income (Loss)	[1]	2.0	(1.7)	16.3	(1.3)
Assets		417.1		417.1		$ 310.1
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue		21.2	22.7	41.4	42.7
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		$ 4.0	$ 3.8	$ 7.9	$ 9.0

[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.)