Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 MWh MW Contracts		Dec. 31, 2011	Dec. 31, 2010 MWh	Dec. 31, 2009 MWh
Long-term Purchase Commitment [Line Items]
Contract expenses	$ 70,188,000		$ 81,828,000	$ 79,598,000
Megawatt Capacity	750
Estimated 2013 Costs	78,084,000
2013 Debt Service Costs	32,966,000
Interest included in 2013 Debt Service Costs	16,018,000
Debt Outstanding	293,952,000
Percentage of Company's energy output (percent)	17.50%
Average cost of Company's energy output (US$ per kWh)	0.018
Number of Public Utility Districts with long term purchase agreements	3
Payment Obligations for Power Purchases
2013	145,874,000
2014	138,311,000
2015	199,606,000
2016	230,789,000
2017	262,582,000
Thereafter	2,364,501,000
Total	3,341,663,000
Total energy obtained during period under purchased power contracts (MWh)	6,100,000			8,500,000	8,200,000
Cost incurred during period to provide energy under purchased power contracts	203,100,000			391,800,000	420,600,000
Daily take obligation under long-term service contract (percent)	100.00%
Daily delivery obligation under long-term service contract (percent)	100.00%
Columbia River projects
Payment Obligations for Power Purchases
2013	65,839,000
2014	66,313,000
2015	65,425,000
2016	65,485,000
2017	65,030,000
Thereafter	693,057,000
Total	1,021,149,000
Other utilities
Payment Obligations for Power Purchases
2013	16,928,000
2014	17,646,000
2015	16,983,000
2016	17,230,000
2017	10,453,000
Thereafter	0
Total	79,240,000
Non-utility contracts
Payment Obligations for Power Purchases
2013	63,107,000
2014	54,352,000
2015	117,198,000
2016	148,074,000
2017	187,099,000
Thereafter	1,671,444,000
Total	2,241,274,000
Gas-fired generation facility
Payment Obligations for Power Purchases
2013	53,400,000
Purchase obligation under longer term (2013 through 2029) agreements	417,800,000
Short-term energy supply contracts
Payment Obligations for Power Purchases
2013	297,000,000
2014	189,600,000
2015	51,300,000
Firm transportation service
Payment Obligations for Power Purchases
2013	160,590,000
2014	157,187,000
2015	144,036,000
2016	139,423,000
2017	136,950,000
Thereafter	555,905,000
Total	1,294,091,000
Deman charges incurred under firm supply and service contracts	142,600,000
Firm storage service
Payment Obligations for Power Purchases
2013	6,135,000
2014	5,209,000
2015	5,209,000
2016	5,209,000
2017	5,209,000
Thereafter	5,247,000
Total	32,218,000
Deman charges incurred under firm supply and service contracts	6,600,000
Firm natural gas supply
Payment Obligations for Power Purchases
2013	166,725,000
2014	162,396,000
2015	149,245,000
2016	144,632,000
2017	142,159,000
Thereafter	561,152,000
Total	1,326,309,000
Combustion turbines
Payment Obligations for Power Purchases
Deman charges incurred under firm supply and service contracts	31,500,000
Energy production service contracts
Payment Obligations for Power Purchases
2013	25,007,000	[1]
2014	25,602,000	[1]
2015	32,148,000	[1]
2016	10,442,000	[1]
2017	10,143,000	[1]
Thereafter	36,364,000	[1]
Total	139,706,000	[1]
Information technology service contracts
Payment Obligations for Power Purchases
2013	13,345,000
2014	0
2015	0
2016	0
2017	0
Thereafter	0
Total	13,345,000
Automated meter reading system
Payment Obligations for Power Purchases
2013	20,480,000	[2]
2014	21,135,000	[2]
2015	21,899,000	[2]
2016	14,241,000	[2]
2017	103,414,000	[2]
Thereafter	0	[2]
Total	181,169,000	[2]
Service contract obligations
Payment Obligations for Power Purchases
2013	58,832,000
2014	46,737,000
2015	54,047,000
2016	24,683,000
2017	113,557,000
Thereafter	36,364,000
Total	334,220,000
Minimum | Combustion turbines
Payment Obligations for Power Purchases
Remaining terms under contract	less than one year
Maximum | Combustion turbines
Payment Obligations for Power Purchases
Remaining terms under contract	32 years
Rock Island Project
Long-term Purchase Commitment [Line Items]
Percent of Output	25.00%
Megawatt Capacity	156
Estimated 2013 Costs	28,601,000
2013 Debt Service Costs	11,206,000
Interest included in 2013 Debt Service Costs	6,635,000
Debt Outstanding	106,546,000
Rocky Reach Project
Long-term Purchase Commitment [Line Items]
Percent of Output	25.00%
Megawatt Capacity	325
Estimated 2013 Costs	25,414,000
2013 Debt Service Costs	8,772,000
Interest included in 2013 Debt Service Costs	3,810,000
Debt Outstanding	62,547,000
Wells Project
Long-term Purchase Commitment [Line Items]
Percent of Output	29.90%
Megawatt Capacity	251
Estimated 2013 Costs	15,983,000
2013 Debt Service Costs	8,548,000
Interest included in 2013 Debt Service Costs	3,079,000
Debt Outstanding	76,925,000
Priest Rapids Development
Long-term Purchase Commitment [Line Items]
Percent of Output	0.80%
Megawatt Capacity	9
Estimated 2013 Costs	4,043,000
2013 Debt Service Costs	2,220,000
Interest included in 2013 Debt Service Costs	1,247,000
Debt Outstanding	23,967,000
Wanapum Development
Long-term Purchase Commitment [Line Items]
Percent of Output	0.80%
Megawatt Capacity	9
Estimated 2013 Costs	4,043,000
2013 Debt Service Costs	2,220,000
Interest included in 2013 Debt Service Costs	1,247,000
Debt Outstanding	$ 23,967,000

[1] Energy production service contracts include operations and maintenance contracts on Mint Farm, Wild Horse, Goldendale electric generating facility (Goldendale), Hopkins Ridge, Frederickson 1, Sumas, Ferndale and Lower Snake River facilities.

[2] Automated meter reading system contractual obligation is the service component of the Landis and Gyr contract.