Retirement Benefits - Net Periodic Benefit Cost (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 16,926	$ 15,822	$ 16,089
Interest cost	25,986	26,263	27,975
Expected return on plan assets	(36,203)	(35,344)	(32,941)
Amortization of prior service cost/(credit)	(1,980)	(1,980)	(165)
Amortization of net loss	768	0	70
Net periodic benefit cost	5,497	4,761	11,028
SERP Pension Benefits
Components of net periodic benefit cost:
Service cost	1,073	1,241	1,024
Interest cost	2,152	2,192	2,165
Amortization of prior service cost/(credit)	0	0	0
Amortization of net loss	702	360	0
Net periodic benefit cost	3,927	3,793	3,189
Other Benefits
Components of net periodic benefit cost:
Service cost	139	113	106
Interest cost	751	806	880
Expected return on plan assets	(435)	(502)	(510)
Amortization of prior service cost/(credit)	0	0	0
Amortization of net loss	53	(46)	(67)
Net periodic benefit cost	508	371	409
PUGET SOUND ENERGY, INC. | Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	16,926	15,822	16,089
Interest cost	25,986	26,263	27,975
Expected return on plan assets	(41,533)	(44,128)	(43,892)
Amortization of prior service cost/(credit)	(1,573)	(1,573)	548
Amortization of net loss	15,015	10,250	7,325
Amortization of transition obligation	0	0	0
Net periodic benefit cost	14,821	6,634	8,045
PUGET SOUND ENERGY, INC. | SERP Pension Benefits
Components of net periodic benefit cost:
Service cost	1,073	1,241	1,024
Interest cost	2,152	2,192	2,165
Expected return on plan assets	0	0	0
Amortization of prior service cost/(credit)	293	563	562
Amortization of net loss	1,432	1,194	769
Amortization of transition obligation	0	0	0
Net periodic benefit cost	4,950	5,190	4,520
PUGET SOUND ENERGY, INC. | Other Postretirement Benefit Plans, Defined Benefit
Components of net periodic benefit cost:
Service cost	139	113	106
Interest cost	751	806	880
Expected return on plan assets	(435)	(502)	(509)
Amortization of prior service cost/(credit)	35	63	132
Amortization of net loss	(245)	(481)	(553)
Amortization of transition obligation	50	50	50
Net periodic benefit cost	$ 295	$ 49	$ 106