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Employee Benefit Plans Investment Strategy (Details)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Target asset allocation allowable range of plus or minus 5.00%  
Other Postretirement Benefit Plan [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 100.00% 100.00%
Cash and Cash Equivalents [Member] | Other Postretirement Benefit Plan [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 1.00% 0.10%
Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 55.00% 55.00%
Debt Securities [Member] | Pension Plan [Member] | International [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 5.00% 5.00%
Debt Securities [Member] | Other Postretirement Benefit Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 40.00% 40.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 37.00% 37.00%
Debt Securities [Member] | Other Postretirement Benefit Plan [Member] | International [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 0.00% 0.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 0.00% 0.00%
Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 34.00% 34.00%
Equity Securities [Member] | Pension Plan [Member] | International [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 6.00% 6.00%
Equity Securities [Member] | Other Postretirement Benefit Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 50.00% 50.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 52.60% 54.20%
Equity Securities [Member] | Other Postretirement Benefit Plan [Member] | International [Member]    
Defined Benefit Plan, Assets, Target Allocations [Abstract]    
Target allocation of investments by plan 10.00% 10.00%
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 9.40% 8.70%
Montana | Pension Plan [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 100.00% 100.00%
Montana | Cash and Cash Equivalents [Member] | Pension Plan [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 0.00% 0.40%
Montana | Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 53.40% 54.90%
Montana | Debt Securities [Member] | Pension Plan [Member] | International [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 4.60% 4.70%
Montana | Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 36.00% 33.90%
Montana | Equity Securities [Member] | Pension Plan [Member] | International [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 6.00% 6.10%
South Dakota | Pension Plan [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 100.00% 100.00%
South Dakota | Cash and Cash Equivalents [Member] | Pension Plan [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 0.10% 0.00%
South Dakota | Debt Securities [Member] | Pension Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 64.40% 65.80%
South Dakota | Debt Securities [Member] | Pension Plan [Member] | International [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 4.40% 4.50%
South Dakota | Equity Securities [Member] | Pension Plan [Member] | Domestic [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 26.00% 24.90%
South Dakota | Equity Securities [Member] | Pension Plan [Member] | International [Member]    
Defined Benefit Plan, Actual Asset Allocations [Abstract]    
Actual allocation of investments by plan 5.10% 4.80%