Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Apr. 16, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term Debt	$ 1,495.2		$ 1,513.3
Unamortized Debt Issuance Costs	(9.2)		(10.0)
Total Long-Term Debt	1,486.0		1,503.3
Less: Due Within One Year	57.5		64.1
Net Long-Term Debt	$ 1,428.5		1,439.2
First Mortgage Bonds - 1.83% Series Due 2018 [Member]
Debt Instrument [Line Items]
Interest Rate	1.83%
Long-term Debt	$ 0.0		50.0
First Mortgage Bonds - 8.17% Series Due 2019 [Member]
Debt Instrument [Line Items]
Interest Rate	8.17%
Long-term Debt	$ 42.0		42.0
First Mortgage Bonds - 5.28% Series Due 2020 [Member]
Debt Instrument [Line Items]
Interest Rate	5.28%
Long-term Debt	$ 35.0		35.0
First Mortgage Bonds - 2.80% Series Due 2020 [Member]
Debt Instrument [Line Items]
Interest Rate	2.80%
Long-term Debt	$ 40.0		40.0
First Mortgage Bonds - 4.85% Series Due 2021 [Member]
Debt Instrument [Line Items]
Interest Rate	4.85%
Long-term Debt	$ 15.0		15.0
First Mortgage Bonds - 3.02% Series Due 2021 [Member]
Debt Instrument [Line Items]
Interest Rate	3.02%
Long-term Debt	$ 60.0		60.0
First Mortgage Bonds - 3.40% Series Due 2022 [Member]
Debt Instrument [Line Items]
Interest Rate	3.40%
Long-term Debt	$ 75.0		75.0
First Mortgage Bonds - 6.02% Series Due 2023 [Member]
Debt Instrument [Line Items]
Interest Rate	6.02%
Long-term Debt	$ 75.0		75.0
First Mortgage Bonds - 3.69% Series Due 2024 [Member]
Debt Instrument [Line Items]
Interest Rate	3.69%
Long-term Debt	$ 60.0		60.0
First Mortgage Bonds - 4.90% Series Due 2025 [Member]
Debt Instrument [Line Items]
Interest Rate	4.90%
Long-term Debt	$ 30.0		30.0
First Mortgage Bonds - 5.10% Series Due 2025 [Member]
Debt Instrument [Line Items]
Interest Rate	5.10%
Long-term Debt	$ 30.0		30.0
First Mortgage Bonds - 3.20% Series Due 2026 [Member]
Debt Instrument [Line Items]
Interest Rate	3.20%
Long-term Debt	$ 75.0		75.0
First Mortgage Bonds - 5.99% Series Due 2027 [Member]
Debt Instrument [Line Items]
Interest Rate	5.99%
Long-term Debt	$ 60.0		60.0
First Mortgage Bonds - 3.30% Series Due 2028 [Member]
Debt Instrument [Line Items]
Interest Rate	3.30%
Long-term Debt	$ 40.0		40.0
First Mortgage Bonds - 3.74% Series Due 2029 [Member]
Debt Instrument [Line Items]
Interest Rate	3.74%
Long-term Debt	$ 50.0		50.0
First Mortgage Bonds - 3.86% Series Due 2030 [Member]
Debt Instrument [Line Items]
Interest Rate	3.86%
Long-term Debt	$ 60.0		60.0
First Mortgage Bonds - 5.69% Series Due 2036 [Member]
Debt Instrument [Line Items]
Interest Rate	5.69%
Long-term Debt	$ 50.0		50.0
First Mortgage Bonds - 6.00% Series Due 2040 [Member]
Debt Instrument [Line Items]
Interest Rate	6.00%
Long-term Debt	$ 35.0		35.0
First Mortgage Bonds - 5.82% Series Due 2040 [Member]
Debt Instrument [Line Items]
Interest Rate	5.82%
Long-term Debt	$ 45.0		45.0
First Mortgage Bonds - 4.08% Series Due 2042 [Member]
Debt Instrument [Line Items]
Interest Rate	4.08%
Long-term Debt	$ 85.0		85.0
First Mortgage Bonds - 4.21% Series Due 2043 [Member]
Debt Instrument [Line Items]
Interest Rate	4.21%
Long-term Debt	$ 60.0		60.0
First Mortgage Bonds - 4.95% Series Due 2044 [Member]
Debt Instrument [Line Items]
Interest Rate	4.95%
Long-term Debt	$ 40.0		40.0
First Mortgage Bonds - 5.05% Series Due 2044 [Member]
Debt Instrument [Line Items]
Interest Rate	5.05%
Long-term Debt	$ 40.0		40.0
First Mortgage Bonds - 4.39% Series Due 2044 [Member]
Debt Instrument [Line Items]
Interest Rate	4.39%
Long-term Debt	$ 50.0		50.0
ALLETE Bonds 4.07% Due April 2048 [Member]
Debt Instrument [Line Items]
Interest Rate	4.07%	4.07%
Long-term Debt	$ 60.0		0.0
Variable Demand Revenue Refunding Bonds Series 1997 A Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	13.5		13.5
ALLETE Term Loan Variable Rate Due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 10.0		40.0
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.025%
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member]
Debt Instrument [Line Items]
Interest Rate	3.26%
Long-term Debt	$ 57.2		65.9
A415SeriesDue2028 [Member]
Debt Instrument [Line Items]
Interest Rate	4.15%
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 27.8		27.8
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member]
Debt Instrument [Line Items]
Interest Rate	3.11%
Long-term Debt	$ 80.0		80.0
SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 15.0		15.0
SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member]
Debt Instrument [Line Items]
Interest Rate	4.14%
Long-term Debt	$ 12.0		0.0
Other Long-Term Debt, 3.11% – 5.75% Due 2019 – 2037 [Member]
Debt Instrument [Line Items]
Long-term Debt	$ 67.7		$ 69.1
Minimum [Member] | Other Long-Term Debt, 3.11% – 5.75% Due 2019 – 2037 [Member]
Debt Instrument [Line Items]
Interest Rate	3.11%
Maximum [Member] | Other Long-Term Debt, 3.11% – 5.75% Due 2019 – 2037 [Member]
Debt Instrument [Line Items]
Interest Rate	5.75%