Utility Plant - Jointly Owned Utility Plant (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Jointly Owned Utility Plant Interests
Asset Retirement Obligation, Liabilities Incurred	$ 7,737		$ 350
Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Company’s Ownership Share	50.00%
Plant in Service at Cost	148,075
Accumulated Depreciation	(6,152)
Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Company’s Ownership Share	25.00%
Plant in Service at Cost	217,899
Accumulated Depreciation	(22,459)
Colstrip Units 1 – 4 Common Facilities
Jointly Owned Utility Plant Interests
Plant in Service at Cost	83	[1]
Accumulated Depreciation	(14)	[1]
Frederickson 1
Jointly Owned Utility Plant Interests
Company’s Ownership Share	49.85%
Plant in Service at Cost	61,779
Accumulated Depreciation	(2,030)
PUGET SOUND ENERGY, INC. | Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Plant in Service at Cost	284,692
Accumulated Depreciation	(142,768)
PUGET SOUND ENERGY, INC. | Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Plant in Service at Cost	496,267
Accumulated Depreciation	(300,827)
PUGET SOUND ENERGY, INC. | Colstrip Units 1 – 4 Common Facilities
Jointly Owned Utility Plant Interests
Plant in Service at Cost	251	[1]
Accumulated Depreciation	(182)	[1]
PUGET SOUND ENERGY, INC. | Frederickson 1
Jointly Owned Utility Plant Interests
Plant in Service at Cost	70,712
Accumulated Depreciation	$ (10,964)

[1] The Company’s ownership is 50% for Colstrip Units 1 & 2 and 25% for Colstrip Units 3 & 4.