Business Segments (Details) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2018 USD ($) a	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) a Segments	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments				3
Operating Revenue		$ 348.0	$ 362.5	$ 1,050.3	$ 1,081.4
Net Income (Loss)		30.7	44.9	113.0	130.8
Assets		5,163.3		$ 5,163.3		$ 5,080.0
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments				3
Operating Revenue		261.3	277.6	$ 789.3	824.1
Net Income (Loss)		29.8	34.2	99.7	110.1
Assets		3,904.3		3,904.3		3,886.6
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue	[1]	30.8	28.3	102.2	95.4
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue	[1]	38.1	36.2	110.9	108.1
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue	[1]	14.2	14.6	41.9	45.1
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue	[1]	117.3	122.3	347.5	365.6
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue	[1]	40.9	41.2	127.3	124.1
Regulated Operations [Member] | CIP Financial Incentive [Member]
Business Segments [Line Items]
Operating Revenue	[1],[2]	3.0	0.0	3.0	5.5
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	[1],[3]	17.0	35.0	56.5	80.3
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		15.0	13.5	57.9	56.8
Net Income (Loss)		1.0	0.6	15.9	11.1
Assets		639.0		639.0		600.5
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue	[1]	9.0	7.6	40.0	39.1
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue	[1]	6.0	5.9	17.9	17.7
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue		45.0	40.2	124.7	110.7
Net Income (Loss)		1.7	1.3	0.5	1.6
Assets		293.7		293.7		292.4
U.S. Water Services [Member] | Point-in-Time [Member]
Business Segments [Line Items]
Operating Revenue	[1]	29.0	26.2	77.0	71.9
U.S. Water Services [Member] | Contract [Member]
Business Segments [Line Items]
Operating Revenue	[1]	9.7	9.1	28.7	26.8
U.S. Water Services [Member] | Capital Project [Member]
Business Segments [Line Items]
Operating Revenue	[1]	$ 6.3	4.9	$ 19.0	12.0
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		$ 26.7	31.2	$ 78.4	89.8
Net Income (Loss)	[4]	(1.8)	8.8	(3.1)	8.0
Assets		326.3		326.3		300.5
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue	[1]	21.2	21.1	63.9	66.4
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue	[1]	$ 5.5	$ 10.1	$ 14.5	23.4
Renewable Cost Recovery Rider [Member] | MPUC [Member] | Retail Customers [Member] | Electric Rates [Member] | Corporate and Other [Member]
Business Segments [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions					7.9	7.9
Renewable Cost Recovery Rider [Member] | MPUC [Member] | Retail Customers [Member] | Minnesota Power [Member] | Electric Rates [Member]
Business Segments [Line Items]
Public Utilities, Approved Rate Increase, Amount					$ 14.3	$ 14.3

[1] With the adoption of new revenue recognition guidance, the Company has enhanced the presentation of business segment Operating Revenue. (See Note 1. Operations and Significant Accounting Policies.)

[2] See Note 6. Regulatory Matters.

[3] Operating revenue in 2017 included $14.3 million of additional revenue as a result of the favorable impact for the regulatory outcome of the MPUC’s modification of its November 2016 order on the allocation of North Dakota investment tax credits. (See Note 6. Regulatory Matters.)

[4] Net income in 2017 included a $7.9 million after-tax favorable impact for the regulatory outcome of the MPUC’s modification of its November 2016 order on the allocation of North Dakota investment tax credits. (See Note 6. Regulatory Matters.)