Summary of Significant Accounting Policies - Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss)
Net unrealized loss on energy derivative instruments	$ (333)	$ (705)
Net unrealized loss on interest rate swaps	0	(94)
Net unrealized loss and prior service cost on pension plans	(36,710)	48,514
Accumulated other comprehensive income (loss), net of tax	(37,043)	47,715
PUGET SOUND ENERGY, INC.
Accumulated Other Comprehensive Income (Loss)
Net unrealized loss on energy derivative instruments	(686)	(2,027)
Net unrealized loss on interest rate swaps	(5,990)	(6,307)
Net unrealized loss and prior service cost on pension plans	(164,281)	(87,405)
Accumulated other comprehensive income (loss), net of tax	$ (170,957)	$ (95,739)