Summary of Significant Accounting Policies Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Regulatory Assets, Current		$ 61,524	$ 69,178	$ 67,702
Regulatory Assets, Noncurrent		351,390	370,711	348,421
Unrealized Loss On Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[1]	17,313	22,092	23,242
Regulatory Assets, Noncurrent	[1]	1,237	3,447	1,117
Asset Recoverable Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current		7,978	8,717	19,653
Regulatory Assets, Noncurrent		2,462	1,949	4,334
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[2],[3]	9,096	9,270	0
Regulatory Assets, Noncurrent	[3]	67,999	76,584	50,175
Environmental Regulatory Assets Noncurrent	[4]	77,095	85,854		$ 50,175
Other Regulatory [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[5]	13,902	10,324	13,111
Regulatory Assets, Noncurrent	[5]	9,789	11,362	10,683
Pension Balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[6]	46,247	43,748	35,374
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		40,106	43,049	44,767
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		180,909	184,223	197,601
Decoupling [Domain]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[7]	13,235	18,775	11,696
Regulatory Assets, Noncurrent	[7]	$ 2,641	$ 6,349	$ 4,370

[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] Environmental costs relate to specific sites approved for regulatory deferral by the OPUC and WUTC. In Oregon, we earn a carrying charge on cash amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. We also accrue a carrying charge on insurance proceeds for amounts owed to customers. In Washington, a carrying charge related to deferred amounts will be determined in a future proceeding. 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 tariff rider. The amounts allocable to Oregon are recoverable through utility rates, subject to an earnings test.

[3] nvironmental costs relate to specific sites approved for regulatory deferral by the OPUC and WUTC. In Oregon, we earn a carrying charge on cash amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. We also accrue a carrying charge on insurance proceeds for amounts owed to customers. In Washington, recovery of deferred amounts will be determined in a future proceeding. Current environmental costs represent remediation costs management expects to collect from Oregon 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 tariff rider. The amounts allocable to Oregon are subject to an earnings test. See Note 13.

[4] Includes pre-review and post-review deferred costs, amounts currently in amortization and interest, net of amounts collected from customers

[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.

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