Summary of Significant Accounting Policies Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory Assets, Current		$ 69,178	$ 68,562
Regulatory Assets, Noncurrent		$ 370,711	368,908
Regulatory Asset, Amortization Period		12 months
Regulatory Asset, Amortization Period, Low End of Range		less than one year
Unrealized Loss On Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[1]	$ 22,092	29,889
Regulatory Assets, Noncurrent	[1]	3,447	3,515
Asset Recoverable Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current		8,717	21,794
Regulatory Assets, Noncurrent		1,949	5,971
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[2],[3]	9,270	0
Regulatory Assets, Noncurrent	[3]	76,584	58,859
Decoupling [Domain]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[4]	18,775	7,505
Other Regulatory [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[5]	10,324	9,374
Regulatory Assets, Noncurrent	[5]	17,711	18,750
Pension Balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[6]	43,748	32,541
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		43,049	47,427
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		$ 184,223	$ 201,845

[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] Current environmental costs represent remediation costs management expects to collect from customers in the next 12 months. Amounts included in this estimate are still subject to a prudence and earnings test review by the OPUC and do not include the $5 million base rate rider. The Oregon amounts are recoverable through utility rates, subject to an earnings test.

[3] Current environmental costs represent remediation costs management expects to collect from customers in the next 12 months. Amounts included in this estimate are still subject to a prudence and earnings test review by the OPUC and do not include the $5 million base rate rider. The amounts allocable to Oregon are recoverable through utility rates, subject to an earnings test. See Note 15.

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

[5] These balances primarily consist of deferrals and amortizations under approved regulatory mechanisms. The accounts being amortized typically earn a rate of return or carrying charge.

[6] The deferral of certain pension expenses above or below the amount set in rates was approved by the OPUC, with recovery of these deferred amounts through the implementation of a balancing account, which includes the expectation of lower net periodic benefit costs in future years. Deferred pension expense balances include accrued interest at the utility’s authorized rate of return, with the equity portion of interest income recognized when amounts are collected in rates.