Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Accounting Policies [Abstract]
Public Utilities Property Plant And Equipment, Disclosure Of Composite Depreciation Rate Number Of Components	3
Weighted Average Depreciation Rates Transmission Distribution	2.70%
Weighted Average Depreciation Rates Gas Storage	2.30%
Weighted Average Depreciation Rates General Plant	4.40%
Weighted Average Depreciation Rates Intangible Other	2.70%
Public Utilities, Allowance for Funds Used During Construction, Rate	5.50%	0.70%	0.40%
Accrued unbilled revenue	$ 62,381	$ 64,946
Revenue Taxes	19,069	17,100	$ 18,000
Inventory - Gas	36,700	42,700
Inventory, Raw Materials and Supplies, Net of Reserves	11,300	11,400
Regulatory Assets, Current	45,781	42,362
Impairment of Long-Lived Assets	$ 192,478	0	$ 0
Regulatory Asset, Amortization Period, Low End of Range	less than one year
Regulatory Assets, Noncurrent	$ 356,608	357,530
Decoupling [Domain]
Regulatory Assets, Current	11,227	13,067
Regulatory Assets, Noncurrent	3,970	5,860
Deferred Income Tax Charge [Member]
Regulatory Assets, Current	2,218	4,378
Regulatory Assets, Noncurrent	$ 19,991	$ 38,670
Minimum [Member]
PGA deferral rate	80.00%
Impact on earnings from gas cost sharing	10.00%
Maximum [Member]
PGA deferral rate	90.00%
Impact on earnings from gas cost sharing	20.00%
OREGON
PGA deferral rate	90.00%	90.00%	80.00%
WASHINGTON
PGA deferral rate	100.00%	100.00%	100.00%
Accounts Payable Location Member [Member]
Bank Overdrafts	$ 4,800	$ 2,900
Utility Plant [Member]
Accounting Policies [Abstract]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.80%	2.80%	2.80%