Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 MW MWh Contracts		Dec. 31, 2013	Dec. 31, 2012 MWh	Dec. 31, 2010 MWh
Long-term Purchase Commitment [Line Items]
Percentage of Company's energy output (percent)	14.90%
Average cost of Company's energy output (US$ per kWh)	0
Number of Public Utility Districts with long term purchase agreements	3
Contract expenses	$ 69,661,000		$ 63,365,000	$ 70,188,000
Megawatt Capacity	749
Estimated 2015 Costs	82,092,000
2015 Debt Service Costs	32,811,000
Interest included in 2015 Debt Service Costs	14,604,000
Debt Outstanding	256,077,000
Payment Obligations for Power Purchases
2014	206,531,000
2015	248,107,000
2016	285,730,000
2017	271,530,000
2018	264,426,000
Thereafter	1,998,857,000
Total	3,275,181,000
Total energy obtained during period under purchased power contracts (MWh)	12,100,000			10,700,000	6,100,000
Cost incurred during period to provide energy under purchased power contracts	401,400,000			348,700,000	203,100,000
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
2014	73,023,000
2015	75,360,000
2016	74,851,000
2017	65,981,000
2018	53,837,000
Thereafter	644,666,000
Total	987,718,000
Other utilities
Payment Obligations for Power Purchases
2014	16,136,000
2015	18,884,000
2016	11,823,000
2017	1,257,000
2018	890,000
Thereafter	0
Total	48,990,000
Non-utility contracts
Payment Obligations for Power Purchases
2014	117,372,000
2015	153,863,000
2016	199,056,000
2017	204,292,000
2018	209,699,000
Thereafter	1,354,191,000
Total	2,238,473,000
Gas-fired generation facility
Payment Obligations for Power Purchases
2014	63,600,000
Purchase obligation under longer term (2013 through 2029) agreements	286,700,000
Short-term energy supply contracts
Payment Obligations for Power Purchases
2014	105,800,000
2015	32,400,000
2016	6,500,000
Firm transportation service
Payment Obligations for Power Purchases
2014	154,121,000
2015	147,424,000
2016	144,751,000
2017	131,484,000
2018	112,249,000
Thereafter	373,958,000
Total	1,063,987,000
Deman charges incurred under firm supply and service contracts	155,000,000
Firm storage service
Payment Obligations for Power Purchases
2014	6,528,000
2015	5,337,000
2016	5,209,000
2017	1,407,000
2018	1,535,000
Thereafter	1,258,000
Total	21,274,000
Deman charges incurred under firm supply and service contracts	6,600,000
Firm natural gas supply
Payment Obligations for Power Purchases
2014	160,649,000
2015	152,761,000
2016	149,960,000
2017	132,891,000
2018	113,784,000
Thereafter	375,216,000
Total	1,085,261,000
Combustion turbines
Payment Obligations for Power Purchases
Deman charges incurred under firm supply and service contracts	35,700,000
Energy production service contracts
Payment Obligations for Power Purchases
2014	32,979,000	[1]
2015	15,650,000	[1]
2016	5,558,000	[1]
2017	4,064,000	[1]
2018	2,372,000	[1]
Thereafter	23,443,000	[1]
Total	84,066,000	[1]
Automated meter reading system
Payment Obligations for Power Purchases
2014	25,402,000	[2]
2015	16,081,000	[2]
2016	13,362,000	[2]
2017	13,996,000	[2]
2018	14,602,000	[2]
Thereafter	51,986,000	[2]
Total	135,429,000	[2]
Service contract obligations
Payment Obligations for Power Purchases
2014	58,381,000
2015	31,731,000
2016	18,920,000
2017	18,060,000
2018	16,974,000
Thereafter	75,429,000
Total	219,495,000
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	30
Rock Island Project
Long-term Purchase Commitment [Line Items]
Percent of Output	25.00%
Megawatt Capacity	156
Estimated 2015 Costs	29,854,000
2015 Debt Service Costs	11,009,000
Interest included in 2015 Debt Service Costs	6,174,000
Debt Outstanding	98,191,000
Rocky Reach Project
Long-term Purchase Commitment [Line Items]
Percent of Output	25.00%
Megawatt Capacity	325
Estimated 2015 Costs	27,583,000
2015 Debt Service Costs	8,245,000
Interest included in 2015 Debt Service Costs	3,343,000
Debt Outstanding	53,299,000
Wells Project
Long-term Purchase Commitment [Line Items]
Percent of Output	29.90%
Megawatt Capacity	251
Estimated 2015 Costs	17,277,000
2015 Debt Service Costs	9,383,000
Interest included in 2015 Debt Service Costs	2,597,000
Debt Outstanding	64,931,000
Priest Rapids Development
Long-term Purchase Commitment [Line Items]
Percent of Output	0.60%
Megawatt Capacity	8
Estimated 2015 Costs	3,689,000
2015 Debt Service Costs	2,087,000
Interest included in 2015 Debt Service Costs	1,245,000
Debt Outstanding	19,828,000
Wanapum Development
Long-term Purchase Commitment [Line Items]
Percent of Output	0.60%
Megawatt Capacity	9
Estimated 2015 Costs	3,689,000
2015 Debt Service Costs	2,087,000
Interest included in 2015 Debt Service Costs	1,245,000
Debt Outstanding	$ 19,828,000

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