Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details)		12 Months Ended
		Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounting Policies [Abstract]
SRRM pre-tax regulatory disallowance		$ 15,000,000
Deferred environmental charges		$ 95,000,000
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		2.20%
Weighted Average Depreciation Rates General Plant		4.20%
Weighted Average Depreciation Rates Intangible Other		2.80%
Public Utilities, Allowance for Funds Used During Construction, Rate		0.70%	0.40%	0.30%
Accrued unbilled revenue		$ 64,946,000	$ 57,987,000
Revenue Taxes		17,111,000	18,034,000	$ 18,837,000
Inventory - Gas		42,700,000	59,300,000
Inventory, Raw Materials and Supplies, Net of Reserves		11,400,000	11,600,000
Unamortized Debt Issuance Expense		$ 7,377,000	7,282,000
Remediation recovery percentage in Oregon		96.68%
Regulatory Assets, Noncurrent		$ 357,530,000	370,711,000
Annual tariff rider collection		5,000,000
SRRM Amortization Expense		13,800,000
Interest disallowed on SRRM		2,800,000.0
Total SRRM disallowance from February 2016 Order		3,300,000.0
SRRM Disallowance, portion attributable to Operations and Maintenance		500,000.0
Decoupling [Domain]
Regulatory Assets, Noncurrent	[1]	5,860,000	6,349,000
Deferred Income Tax Charge [Member]
Regulatory Assets, Noncurrent		38,670,000	43,049,000
Deferred Tax Liabilities, Deferred Expense		$ 43,000,000	$ 47,400,000
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%	80.00%	90.00%
WASHINGTON
PGA deferral rate		100.00%	100.00%	100.00%
Accounts Payable Location Member [Member]
Bank Overdrafts		$ 2,900,000	$ 2,500,000
Gill Ranch Gas Storage Facility Member [Member]
Fair Value, Net Asset (Liability)		$ 196,900,000

[1] This deferral represents the margin adjustment resulting from differences between actual and expected volumes.