Mountain States Transmission Intertie (MSTI) Impairment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012 watts	Sep. 30, 2011	Sep. 30, 2012 watts	Sep. 30, 2011
Mountain States Transmission Intertie (MSTI) Impairment [Abstract]
Megawatts of capacity	1,500		1,500
Mountain States Transmission Intertie impairment	$ 24,039	$ 0	$ 24,039	$ 0

The adjustment to expense of previously capitalized costs when the entity obtains information that raises substantial doubt about the economic or operational viability of the project.

No definition available.

Name:	nwe_ImpairmentOfAbandonedProject
Namespace Prefix:	nwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration