Litigation (Details) - Subsidiaries [Member]	Dec. 31, 2019 unit	Dec. 10, 2019 USD ($) MW
Colstrip Unit 4 [Member]
Jointly Owned Utility Plant Interests
Company’s Ownership Share	25.00%
Colstrip 4 Sale Amount \| $		$ 1
Colstrip PPA \| MW		90
Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Company’s Ownership Share	50.00%
Number of Units \| unit	2

The proportionate share of interest (percent) in a jointly owned utility plant.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant