Commitments and Contingencies (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) MWh Contracts MW	Dec. 31, 2018 USD ($) MWh	Dec. 31, 2017 USD ($) MWh
Long-term Purchase Commitment [Line Items]
Long-term Line of Credit, Noncurrent	$ 24,100,000	$ 11,900,000
Average cost of Company's energy output (US$ per kWh)	$ 0.033
Number of Public Utility Districts with long term purchase agreements | Contracts	3
Contract expenses	$ 87,135,000	$ 80,165,000	$ 73,827,000
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased	10.20%
Long-term Contract for Purchase of Electric Power, Capacity | MW	722
Long-term Contract for Purchase of Electric Power, Estimated Annual Cost	$ 112,036,000
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required	18,609,000
Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges	8,982,000
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	155,859,000
Payment Obligations for Power Purchases
2020	574,442,000
2021	436,821,000
2022	409,907,000
2023	411,265,000
2024	418,569,000
Thereafter	1,579,295,000
Total	$ 3,830,299,000
Total energy obtained during period under purchased power contracts (MWh) | MWh	12.5	14.1	14.5
Cost incurred during period to provide energy under purchased power contracts	$ 550,600,000	$ 508,200,000	$ 456,400,000
Daily take obligation under long-term service contract (percent)	100.00%
Daily delivery obligation under long-term service contract (percent)	100.00%
Natural Gas [Member]
Payment Obligations for Power Purchases
2020	$ 273,263,000
2021	196,806,000
2022	178,208,000
2023	148,165,000
2024	82,509,000
Thereafter	0
Total	878,951,000
Colombia River Projects [Member]
Payment Obligations for Power Purchases
2020	121,680,000
2021	111,125,000
2022	103,879,000
2023	103,377,000
2024	102,976,000
Thereafter	609,912,000
Non-Utility Generators [Member]
Payment Obligations for Power Purchases
Total	216,913,000
Firm Transportation Service [Member]
Payment Obligations for Power Purchases
2020	176,741,000
2021	173,133,000
2022	172,190,000
2023	161,508,000
2024	116,842,000
Thereafter	828,136,000
Total	1,628,550,000
Firm Storage and Peaking Service [Member]
Payment Obligations for Power Purchases
2020	8,954,000
2021	4,503,000
2022	3,014,000
2023	853,000
2024	140,000
Thereafter	213,000
Total	17,677,000
Long-term Purchase Commitment, Demand Charges	125,100,000
Combustion turbines
Payment Obligations for Power Purchases
Long-term Purchase Commitment, Demand Charges	51,200,000
Energy production service contracts
Payment Obligations for Power Purchases
2020	28,474,000
2021	29,219,000
2022	29,923,000
2023	30,645,000
2024	31,400,000
Thereafter	141,817,000
Total	291,478,000
Automated meter reading system
Payment Obligations for Power Purchases
2020	43,971,000
2021	44,849,000
2022	45,526,000
2023	46,218,000
2024	46,926,000
Thereafter	96,149,000
Total	323,639,000
Service contract obligations
Payment Obligations for Power Purchases
2020	72,445,000
2021	74,068,000
2022	75,449,000
2023	76,863,000
2024	78,326,000
Thereafter	237,966,000
Total	615,117,000
Colombia River Projects [Member]
Payment Obligations for Power Purchases
Total	1,152,949,000
Other Utilities [Member]
Payment Obligations for Power Purchases
Total	2,460,437,000
Electric Portfolio
Payment Obligations for Power Purchases
2020	263,940,000
2021	300,795,000
2022	302,838,000
2023	307,888,000
2024	315,593,000
Thereafter	969,383,000
Firm natural gas supply
Payment Obligations for Power Purchases
2020	458,958,000
2021	374,442,000
2022	353,412,000
2023	310,526,000
2024	199,491,000
Thereafter	828,349,000
Total	$ 2,525,178,000
Minimum | Combustion turbines
Payment Obligations for Power Purchases
Remaining terms under contract	1 year
Maximum | Combustion turbines
Payment Obligations for Power Purchases
Remaining terms under contract	25
Rock Island Project
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased	25.00%
Long-term Contract for Purchase of Electric Power, Capacity | MW	156
Long-term Contract for Purchase of Electric Power, Estimated Annual Cost	$ 34,180,000
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required	11,499,000
Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges	5,681,000
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	$ 96,956,000
Rocky Reach Project
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased	25.00%
Long-term Contract for Purchase of Electric Power, Capacity | MW	325
Long-term Contract for Purchase of Electric Power, Estimated Annual Cost	$ 31,190,000
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required	4,940,000
Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges	2,129,000
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	$ 33,317,000
Wells Project [Member]
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased	27.10%
Long-term Contract for Purchase of Electric Power, Capacity | MW	228
Long-term Contract for Purchase of Electric Power, Estimated Annual Cost	$ 43,004,000
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required	0
Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges	0
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	$ 0
Priest Rapids Development
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased	0.60%
Long-term Contract for Purchase of Electric Power, Capacity | MW	6
Long-term Contract for Purchase of Electric Power, Estimated Annual Cost	$ 1,831,000
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required	1,085,000
Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges	586,000
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	$ 12,793,000
Wanapum Development
Long-term Purchase Commitment [Line Items]
Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased	0.60%
Long-term Contract for Purchase of Electric Power, Capacity | MW	7
Long-term Contract for Purchase of Electric Power, Estimated Annual Cost	$ 1,831,000
Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required	1,085,000
Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges	586,000
Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding	12,793,000
Electricity, US Regulated [Member]
Payment Obligations for Power Purchases
2020	188,822,000
2021	24,901,000
2022	3,190,000
2023	0
2024	0
Thereafter	$ 0