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Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	Aug. 25, 2017	Dec. 31, 2017	Jun. 01, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt		$ 1,513.3		$ 1,569.1
Unamortized Debt Issuance Costs		(10.0)		(11.0)
Total Long-Term Debt		1,503.3		1,558.1
Less: Due Within One Year		64.1		187.7
Net Long-Term Debt		$ 1,439.2		1,370.4
First Mortgage Bonds - 1.83% Series Due 2018 [Member]
Debt Instrument [Line Items]
Interest Rate		1.83%
Long-term Debt		$ 50.0		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
Unsecured Term Loan Variable Rate Due 2017 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 0.0		125.0
Senior Unsecured Notes 5.99% Due 2017 [Member]
Debt Instrument [Line Items]
Interest Rate		5.99%
Long-term Debt		$ 0.0		50.0
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]
Variable Rate Basis	LIBOR
Basis Spread on Variable Rate	1.025%
Long-term Debt		$ 40.0		0.0
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member]
Debt Instrument [Line Items]
Interest Rate		3.26%
Long-term Debt		$ 65.9		74.6
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%	3.11%
Long-term Debt		$ 80.0		0.0
SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member]
Debt Instrument [Line Items]
Interest Rate		4.15%
Long-term Debt		$ 15.0		15.0
Other Long-Term Debt, 3.11% – 5.37% Due 2018 – 2037 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 69.1		$ 61.2
Minimum [Member] | Other Long-Term Debt, 3.11% – 5.37% Due 2018 – 2037 [Member]
Debt Instrument [Line Items]
Interest Rate		3.11%
Maximum [Member] | Other Long-Term Debt, 3.11% – 5.37% Due 2018 – 2037 [Member]
Debt Instrument [Line Items]
Interest Rate		5.37%

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