Employee Benefit Plans Cash Flows (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 Pension Plans, Defined Benefit [Member]	Dec. 31, 2012 Northwestern Energy Pension Plan Montana [Member]	Dec. 31, 2011 Northwestern Energy Pension Plan Montana [Member]	Dec. 31, 2010 Northwestern Energy Pension Plan Montana [Member]	Dec. 31, 2012 Northwestern Pension Plan South Dakota [Member]	Dec. 31, 2011 Northwestern Pension Plan South Dakota [Member]	Dec. 31, 2010 Northwestern Pension Plan South Dakota [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension contributions	$ 11,700,000	$ 11,700,000	$ 10,000,000		$ 10,500,000	$ 10,500,000	$ 9,000,000	$ 1,200,000	$ 1,200,000	$ 1,000,000
Expected contributions to pension plans				$ 11,700,000