Employee Benefit Plans Investment Strategy (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation allowable range of plus or minus	5.00%
Northwestern Energy Pension Plan Montana [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Northwestern Pension Plan South Dakota [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Northwestern Energy Health And Welfare [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	100.00%	100.00%
Cash and Cash Equivalents [Member] | Northwestern Energy Pension Plan Montana [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Cash and Cash Equivalents [Member] | Northwestern Pension Plan South Dakota [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Cash and Cash Equivalents [Member] | Northwestern Energy Health And Welfare [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	3.40%	2.00%
Debt Securities [Member] | Pension Plans, Defined Benefit [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	40.00%	40.00%
Debt Securities [Member] | Pension Plans, Defined Benefit [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	10.00%	10.00%
Debt Securities [Member] | Northwestern Energy Pension Plan Montana [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	39.50%	39.50%
Debt Securities [Member] | Northwestern Energy Pension Plan Montana [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	9.90%	10.60%
Debt Securities [Member] | Northwestern Pension Plan South Dakota [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	38.30%	38.40%
Debt Securities [Member] | Northwestern Pension Plan South Dakota [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	10.60%	11.20%
Debt Securities [Member] | Northwestern Energy Health And Welfare [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	37.80%	39.40%
Debt Securities [Member] | Northwestern Energy Health And Welfare [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	0.00%	0.00%
Debt Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	40.00%	40.00%
Debt Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	0.00%	0.00%
Equity Securities [Member] | Pension Plans, Defined Benefit [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	40.00%	40.00%
Equity Securities [Member] | Pension Plans, Defined Benefit [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	10.00%	10.00%
Equity Securities [Member] | Northwestern Energy Pension Plan Montana [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	40.20%	40.30%
Equity Securities [Member] | Northwestern Energy Pension Plan Montana [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	10.40%	9.60%
Equity Securities [Member] | Northwestern Pension Plan South Dakota [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	40.60%	40.90%
Equity Securities [Member] | Northwestern Pension Plan South Dakota [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	10.50%	9.50%
Equity Securities [Member] | Northwestern Energy Health And Welfare [Member] | Domestic [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	49.80%	49.80%
Equity Securities [Member] | Northwestern Energy Health And Welfare [Member] | International [Member]
Defined Benefit Plan, Actual Asset Allocations [Abstract]
Actual allocation of investments by plan	9.00%	8.80%
Equity Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	50.00%	50.00%
Equity Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Target allocation of investments by plan	10.00%	10.00%