Fair Value - Fair Value of Financial Instruments and Assets Measured on a Nonrecurring Basis (Details) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Fair Value of Financial Instruments [Line Items]
Short-Term and Long-Term Debt - Carrying Amount	[1]	$ 2,022.5	$ 1,622.6
ALLETE Clean Energy [Member]
Fair Value of Financial Instruments [Line Items]
Asset Carrying Value - Undiscounted Cash Flow Analysis		11.0
Level 2 [Member]
Fair Value of Financial Instruments [Line Items]
Short-Term and Long-Term Debt - Fair Value	[1]	$ 2,322.1	$ 1,791.8

[1] Excludes unamortized debt issuance costs.