Summary of Significant Accounting Policies Regulatory Assets Disclosure (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Accounting Policies [Abstract]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.80%	2.80%		2.80%
Weighted Average Depreciation Rates Transmission Distribution	2.70%
Weighted Average Depreciation Rates Utility Storage	2.20%
Weighted Average Depreciation Rates General Plant	4.70%
Weighted Average Depreciation Rates Intangible Other	4.80%
Public Utilities, Allowance for Funds Used During Construction, Rate	0.30%	0.50%		0.60%
Outstanding Checks In Accounts Payable	$ 2,300,000	$ 3,900,000
Unbilled Receivables, Current	56,955,000	61,925,000
Senate Bill Four Zero Eight Charge		7,400,000
Inventory - Gas	58,800,000	65,600,000
Inventory - Materials and supplies	8,800,000	8,800,000
Deferred Tax Liabilities, Deferred Expense	60,300,000	68,500,000
Table Of Regulatory Assets [Abstract]
Regulatory Assets, Current	52,448,000	94,673,000
Regulatory Assets, Noncurrent	387,888,000	371,392,000
Unrealized Loss On Derivatives [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets, Current	10,796,000	57,317,000	[1]
Regulatory Assets, Noncurrent	578,000	6,536,000	[1]
Pension and Other Postretirement Benefit Liabilities [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets, Current	17,247,000	15,491,000	[2]
Regulatory Assets, Noncurrent	182,688,000	170,512,000	[2]
Other Regulatory [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets, Current	24,405,000	21,865,000	[3]
Regulatory Assets, Noncurrent	7,239,000	17,402,000	[3]
Pension Balancing [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets, Noncurrent	15,022,000	6,008,000	[2]
Income Tax Asset [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets, Noncurrent	55,879,000	65,264,000
Environmental Restoration Costs [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets, Noncurrent	$ 126,482,000	$ 105,670,000	[4]

[1] Unrealized gains or losses on derivatives are non-cash items and therefore do not earn a rate of return or a carrying charge. These amounts are recoverable through utility rates as part of the annual Purchased Gas Adjustment (PGA) mechanism when realized at settlement.

[2] Certain pension costs of the utility are approved for regulatory deferral, including amounts recorded to the pension balancing account, to mitigate the effects of higher and lower pension expenses. Pension costs that are deferred include an interest component when recognized in net periodic benefit costs or earn a rate of return or carrying charge. See Note 8.

[3] Other primarily consists of several deferrals and amortizations under other approved regulatory mechanisms. The accounts being amortized typically earn a rate of return or carrying charge.

[4] Environmental costs relate to specific sites approved for regulatory deferral. In Oregon we earn a rate of return on amounts paid, whereas amounts accrued but not yet paid do not earn a rate of return or a carrying charge until expended. Environmental costs related to Washington were deferred beginning in 2011, with cost recovery and a carrying charge to be determined in a future proceeding. In the 2012 rate case, the OPUC authorized a Site Remediation and Recovery Mechanism (SRRM) that allows the Company to recover prudently incurred environmental costs, subject to an earnings test that will be defined in a future rate proceeding.