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Pension and Other Postretirement Benefit Plans - Contributions (Details) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
 12 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Mar. 31, 2014
Pension [Member]
Dec. 31, 2014
Pension [Member]
Dec. 31, 2013
Pension [Member]
Dec. 31, 2012
Pension [Member]
Dec. 31, 2014
Postretirement Health and Life [Member]
Dec. 31, 2013
VEBA [Member]
Dec. 31, 2012
VEBA [Member]
Dec. 31, 2013
Grantor Trust [Member]
Dec. 31, 2012
Grantor Trust [Member]
Defined Benefit Plan Disclosure [Line Items]                        
Defined Benefit Plans, Employer Contributions           $ 0 $ 7.3   $ 10.8 $ 1.5 $ 2.0 $ 0
ALLETE Common Stock Contributed to Pension Plan 0 0 20.0 19.5                
ALLETE Common Stock Contributed to Pension Plan, Shares       400                
Defined Benefit Plans, Estimated Employer Contributions         We do not expect to make any additional contributions to the defined benefit pension plan in 2014, beyond the $19.5 million contribution to the defined benefit pension plan made in January 2014.              
Defined Benefit Plans, Estimated Employer Contributions               0        
Defined Contribution Plan, Employer Contributions $ 8.4 $ 7.7