Utility Plant - Jointly Owned Utility Plant (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Jointly Owned Utility Plant Interests
Asset Retirement Obligation, Liabilities Incurred	$ 0	$ 350
Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Company’s Ownership Share	50.00%
Plant in Service at Cost	158,516
Accumulated Depreciation	(12,047)
Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Company’s Ownership Share	25.00%
Plant in Service at Cost	234,787
Accumulated Depreciation	(28,763)
Colstrip Units 1 – 4 Common Facilities
Jointly Owned Utility Plant Interests
Plant in Service at Cost	83
Accumulated Depreciation	(21)
Frederickson 1
Jointly Owned Utility Plant Interests
Company’s Ownership Share	49.85%
Plant in Service at Cost	61,771
Accumulated Depreciation	(5,750)
Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Company’s Ownership Share	33.34%
Plant in Service at Cost	28,185
Accumulated Depreciation	(4,099)
PUGET SOUND ENERGY, INC. \| Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Plant in Service at Cost	293,103
Accumulated Depreciation	(146,635)
PUGET SOUND ENERGY, INC. \| Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Plant in Service at Cost	509,619
Accumulated Depreciation	(303,594)
PUGET SOUND ENERGY, INC. \| Colstrip Units 1 – 4 Common Facilities
Jointly Owned Utility Plant Interests
Plant in Service at Cost	252
Accumulated Depreciation	(189)
PUGET SOUND ENERGY, INC. \| Frederickson 1
Jointly Owned Utility Plant Interests
Plant in Service at Cost	70,719
Accumulated Depreciation	(14,698)
PUGET SOUND ENERGY, INC. \| Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	42,494
Accumulated Depreciation	$ (18,409)

Amount of asset retirement obligations incurred during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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- Definition

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-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=6500719&loc=d3e659983-123033

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section C

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

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- Definition

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

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