Property, Plant and Equipment (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Construction Work in Progress	[1]	$ 0.0		$ 48.2
Property, Plant and Equipment – Net		3,669.1		3,284.8
Asset Retirement Obligation [Roll Forward]
Beginning Obligation		109.2		81.8
Accretion		7.3		5.5
Liabilities Recognized		5.1	[2]	23.0	[3]
Liabilities Settled		(2.6)		(0.5)
Revisions in Estimated Cash Flows		12.4		(0.6)
Ending Obligation		131.4		109.2
Regulated Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment		4,336.7		3,903.3
Construction Work in Progress		101.2		355.4
Accumulated Depreciation		(1,323.8)		(1,260.2)
Property, Plant and Equipment – Net		$ 3,114.1		2,998.5
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		$ 467.3		203.7
Construction Work in Progress		4.0		1.3
Accumulated Depreciation		(24.0)		(8.4)
Property, Plant and Equipment – Net		$ 447.3		196.6
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		44 years
U.S. Water Services [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment		$ 15.6		0.0
Accumulated Depreciation		(3.4)		0.0
Property, Plant and Equipment – Net		$ 12.2		0.0
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	[4]	$ 165.6		152.5
Construction Work in Progress		4.5		4.6
Accumulated Depreciation		(74.6)		(67.4)
Property, Plant and Equipment – Net		$ 95.5		$ 89.7
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		39 years

[1] Construction Costs for Development Project related to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)

[2] The increase in 2015 is related to the ALLETE Clean Energy wind energy facilities acquisitions in 2015. (See Note 7. Acquisitions.)

[3] The increase in 2014 is related to BNI Energy for coal mining expansion and ALLETE Clean Energy due to wind energy facilities acquisitions.(See Note 7. Acquisitions.)

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