Accounting for Derivative Instruments and Hedging Activities Derivative Assets and Liabilities (Details) (USD $)	Mar. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Derivative [Line Items]
Derivative Assets, Current	$ 20,410,000		$ 6,869,000
Assets, Long-term	14,452,000		14,814,000
Derivative Liabilities, Current	81,198,000		177,519,000
Liabilities, Long-term	51,987,000		83,276,000
PUGET SOUND ENERGY, INC. [Member]
Derivative [Line Items]
Derivative Assets, Current	20,410,000		6,869,000
Assets, Long-term	14,452,000		14,814,000
Derivative Liabilities, Current	74,652,000		170,948,000
Liabilities, Long-term	38,567,000		68,323,000
Offsetting regulatory assets	32,200,000				96,000,000
Commodity Contract [Member]
Derivative [Line Items]
Assets	34,862,000		21,683,000
Liabilities	113,219,000		239,271,000
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Assets	34,862,000	[1]	21,683,000	[1]
Liabilities	133,185,000	[2]	260,795,000	[2]
Not Designated as Hedging Instrument [Member] | PUGET ENERGY [Member]
Derivative [Line Items]
Derivative Assets, Current	20,410,000	[1]	6,869,000	[1]
Assets, Long-term	14,452,000	[1]	14,814,000	[1]
Derivative Liabilities, Current	81,198,000	[2]	177,519,000	[2]
Liabilities, Long-term	51,987,000	[2]	83,276,000	[2]
Not Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Derivative [Line Items]
Derivative, Notional Amount	450,000,000	[3]	450,000,000	[3]
Assets	0	[1],[3]	0	[1],[3]
Liabilities	19,966,000	[2],[3]	21,524,000	[2],[3]
Not Designated as Hedging Instrument [Member] | Electric portfolio [Member]
Derivative [Line Items]
Assets	16,132,000	[1]	9,557,000	[1]
Liabilities	62,240,000	[2]	131,193,000	[2]
Not Designated as Hedging Instrument [Member] | Natural gas portfolio [Member]
Derivative [Line Items]
Assets	18,730,000	[1],[4]	12,126,000	[1],[4]
Liabilities	$ 50,979,000	[2],[4]	$ 108,078,000	[2],[4]
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	475,822,952	[4]	516,909,006	[4]

[1] Balance sheet location: Current and Long-term Unrealized gain on derivative instruments.

[2] Balance sheet location: Current and Long-term Unrealized loss on derivative instruments.

[3] Interest rate swap contracts are only held at Puget Energy.

[4] PSE had a net derivative liability and an offsetting regulatory asset of $32.2 million at March 31, 2013 and $96.0 million at December 31, 2012 related to contracts used to economically hedge the cost of physical gas purchased to serve natural gas customers. All fair value adjustments on derivatives relating to the natural gas business have been reclassified to a deferred account in accordance with ASC 980, “Regulated Operations” (ASC 980) due to the Purchased Gas Adjustment (PGA) mechanism.