Property, Plant and Equipment (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment – Net		$ 3,741.2		$ 3,669.1
Asset Retirement Obligation [Roll Forward]
Beginning Obligation		131.4		109.2
Accretion		8.0		7.3
Liabilities Recognized	[1]			5.1
Liabilities Settled		(6.5)		(2.6)
Revisions in Estimated Cash Flows		3.7		12.4
Ending Obligation		136.6		131.4
Regulated Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service		4,437.0		4,336.7
Construction Work in Progress		84.2		101.2
Accumulated Depreciation		(1,426.1)		(1,323.8)
Property, Plant and Equipment – Net		$ 3,095.1		3,114.1
Regulated Operations [Member] | Generation [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		10 years
Regulated Operations [Member] | Generation [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		50 years
Regulated Operations [Member] | Transmission [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		44 years
Regulated Operations [Member] | Transmission [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		67 years
Regulated Operations [Member] | Distribution [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		18 years
Regulated Operations [Member] | Distribution [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		65 years
ALLETE Clean Energy [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service		$ 472.3		467.3
Construction Work in Progress		101.0	[2]	4.0
Accumulated Depreciation		(41.0)		(24.0)
Property, Plant and Equipment – Net		$ 532.3		447.3
ALLETE Clean Energy [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		5 years
ALLETE Clean Energy [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		35 years
U.S. Water Services [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service		$ 19.5		15.6
Accumulated Depreciation		(6.9)		(3.4)
Property, Plant and Equipment – Net		$ 12.6		12.2
U.S. Water Services [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		3 years
U.S. Water Services [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		39 years
Corporate and Other [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service	[3]	$ 179.8		165.6
Construction Work in Progress	[3]	2.8		4.5
Accumulated Depreciation	[3]	(81.4)		(74.6)
Property, Plant and Equipment – Net		$ 101.2		$ 95.5
Corporate and Other [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		3 years
Corporate and Other [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		47 years

[1] The increase in 2015 is related to the ALLETE Clean Energy wind energy facilities acquisitions in 2015. (See Note 6. Acquisitions.)

[2] The increase in ALLETE Clean Energy’s construction work in progress primarily relates to deposits for WTGs. The WTGs will be utilized as ALLETE Clean Energy develops future projects.

[3] Primarily includes BNI Energy and a small amount of non-rate base generation.