Commitments, Guarantees and Contingencies (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011 mW kV Mi mWh	Jun. 30, 2011 North Dakota Wind Development Bison 1 [Member] mW	Jun. 30, 2011 North Dakota Wind Development Bison 2 [Member] mW	Jun. 30, 2011 North Dakota Wind Development Bison 3 [Member] mW	Jun. 30, 2011 BNI Coal Reclamation Project [Member]	Jun. 30, 2011 Allete Properties Performance and Maintenance Obligations [Member]	Jun. 30, 2011 Town Center Community Development District [Member]	Dec. 31, 2010 Town Center Community Development District [Member]	Jun. 30, 2011 Palm Coast Park Community Development District [Member]	Dec. 31, 2010 Palm Coast Park Community Development District [Member]
Square Butte PPA
Square Butte expiration date	Dec. 31, 2026
Coal-fired generating unit capacity	455
Total debt outstanding for power cooperative corporation	$ 424.1
Annual debt service for power cooperative corporation	39.0
Portion of debt service obligation on debt for power cooperative corporation (in hundredths)	50.00%
Minnkota Power Sales Agreement
DC transmission line capacity	250
Minnkota Power expiration date	Dec. 31, 2025
DC transmission line length	465
Wind PPAs
Number of long-term wind PPAs	2
Oliver I capacity	50
Oliver II capacity	48
Length of wind PPAs	25Y
Hydro PPAs
Manitoba Hydro expiration date	Apr. 30, 2015
Manitoba Hydro capacity	50
Manitoba Hydro surplus energy start date	May 01, 2011
Manitoba Hydro surplus energy end date	Apr. 30, 2022
Minimum surplus energy	1,000,000
North Dakota Wind Development
Total expected capital cost		177.0	160.0	160.0
Total capital costs spent		137.4
Total wind development capacity		82	105	105
Length of phase one transmission line	22
Capacity of phase one transmission line	230
Number of phase one turbines	16
Capacity of phase one turbines	2.3
Number of phase two turbines	15
Capacity of phase two turbines	3.0
Leasing Agreements
Leasing agreement expiration date	Dec. 31, 2027
Dragline lease	2.8
Termination fee	3.0
Operating leases, future minimum payment obligation, current year	8.1
Operating leases, future minimum payment obligation, in two years	8.4
Operating leases, future minimum payment obligation, in three years	8.5
Operating leases, future minimum payment obligation, in four years	8.7
Operating leases, future minimum payment obligation, in five years	8.4
Operating leases, future minimum payment obligation, thereafter	44.7
Coal, Rail and Shipping Contracts
Minimum annual payment obligations for purchase contracts, current year	31.8
Minimum annual payment obligations for purchase contracts, year two	15.8
Minimum annual payment obligations for purchase contracts, year three	16.3
Transmission
Amount spent for transmission line projects	17.5
Minimum amount expected to be invested in transmission line projects	100.0
Maximum amount expected to be invested in transmission line projects	125.0
Length of Fargo to Monticello transmission line	238
Capacity of Fargo to Monticello transmission line	345
Length of Bemidji to Grand Rapids transmission line	70
Capacity of Bemidji to Grand Rapids line	230
Solid And Hazardous Waste
Amount of liability for manufactured gas site	0.5
Other
Total current estimated reclamation liability	8.0
Interest rate on tax-exempt capital improvement revenue bonds (in hundredths)							6.00%		5.70%
Ownership percentage of benefited property							73.00%	69.00%	93.00%	93.00%
Annual assessments on projects at current ownership level							1.5		2.2
Surety bonds					29.7	10.2
Capital improvement and special assessment bonds (dollars)							26.4		31.8
Capital improvement and special assessment bonds payable	31Y
Legal Proceedings
Amount of damages being sought from Company related to lawsuit from fire as a result of failure of certain electrical protective equipment	$ 20.0