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Long-Term Debt and Capital Leases Schedule of Long-Term Debt (Details) - Secured Debt - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Jun. 30, 2016
Sep. 30, 2015
Jun. 30, 2015
Sep. 30, 2016
Dec. 31, 2016
Debt Instrument [Line Items]            
Debt issued by third party $ 144.7       $ 144.7  
Montana 2.00% Due 2023            
Debt Instrument [Line Items]            
Debt Instrument, issuance date Aug. 11, 2016          
Interest rate, stated percentage 2.00%       2.00% 2.00%
Maturity date Aug. 01, 2023         Aug. 01, 2023
Montana 4.65%, Due 2023            
Debt Instrument [Line Items]            
Debt instrument, face amount $ 144.7       $ 144.7  
Interest rate, stated percentage 4.65%       4.65% 4.65%
Maturity date Aug. 01, 2023         Aug. 01, 2023
Repurchased face amount, debt instrument $ 170.2       $ 170.2  
South Dakota, 2.80%, Due 2026            
Debt Instrument [Line Items]            
Debt instrument, face amount   $ 60.0        
Debt Instrument, issuance date   Jun. 15, 2016        
Interest rate, stated percentage   2.80%       2.80%
Maturity date   Jun. 15, 2026       Jun. 15, 2026
South Dakota, 6.05%, Due 2018            
Debt Instrument [Line Items]            
Interest rate, stated percentage   6.05%       6.05%
Maturity date   May 01, 2018       May 01, 2018
Repurchased face amount, debt instrument   $ 55.0        
South Dakota, 2.66%, Due 2026            
Debt Instrument [Line Items]            
Debt instrument, face amount $ 45.0       $ 45.0  
Interest rate, stated percentage 2.66%       2.66% 2.66%
Maturity date         Sep. 30, 2026 Sep. 30, 2026
South Dakota, 4.26% Due 2040            
Debt Instrument [Line Items]            
Debt instrument, face amount     $ 70.0      
Debt Instrument, issuance date     Sep. 01, 2015      
Interest rate, stated percentage     4.26%     4.26%
Maturity date     Sep. 29, 2040     Sep. 29, 2040
Secured Debt Montana Due 2025 and 2045 [Member]            
Debt Instrument [Line Items]            
Debt instrument, face amount       $ 200.0    
Debt Instrument, issuance date       Jun. 01, 2015    
Montana 3.11%, Due 2025            
Debt Instrument [Line Items]            
Debt instrument, face amount       $ 75.0    
Interest rate, stated percentage       3.11%   3.11%
Maturity date       Jul. 01, 2025   Jul. 01, 2025
Montana 4.11%, due 2045            
Debt Instrument [Line Items]            
Debt instrument, face amount       $ 125.0    
Interest rate, stated percentage       4.11%   4.11%
Maturity date       Jul. 01, 2045   Jul. 01, 2045
Montana, 6.04%, Due 2016            
Debt Instrument [Line Items]            
Interest rate, stated percentage       6.04%    
Maturity date       Sep. 01, 2016    
Repurchased face amount, debt instrument       $ 150.0