Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Accounting Policies [Abstract]
SRRM pre-tax regulatory disallowance	$ 15,000
Deferred environmental charges	95,000
Interest income from deferred environmental expenses	9,100
After-tax regulatory disallowance on insurance proceeds	$ 5,300
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	4.60%
Weighted Average Depreciation Rates Intangible Other	2.70%
Public Utilities, Allowance for Funds Used During Construction, Rate	0.40%	0.30%	0.30%
Accrued unbilled revenue	$ 57,987	$ 57,963
Revenue Taxes	18,034	18,837	$ 19,002
Inventory - Gas	59,300	68,000
Inventory, Raw Materials and Supplies, Net of Reserves	11,600	9,800
Annual tariff rider collection	5,000
Deferred Income Tax Charge [Member]
Deferred Tax Liabilities, Deferred Expense	$ 47,400	$ 51,800
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	80.00%	90.00%	90.00%
WASHINGTON
PGA deferral rate	100.00%	100.00%	100.00%
Accounts Payable Location Member [Member]
Bank Overdrafts	$ 2,500	$ 5,500