Summary of Significant Accounting Policies Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Regulatory Assets [Line Items]
Regulatory Liability, Current		$ 33,211	$ 40,290	$ 35,596
Regulatory Assets, Current		34,874	42,362	61,524
Regulatory Assets, Noncurrent		349,057	357,530	351,390
Regulatory Liability, Noncurrent		357,587	349,319	346,761
Unrealized Loss On Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[1]	1,580	1,315	17,313
Regulatory Assets, Noncurrent	[1]	2,546	913	1,237
Asset Recoverable Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current		2,757	6,830	7,978
Regulatory Assets, Noncurrent		114	89	2,462
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[2],[3]	7,574	9,989	9,096
Regulatory Assets, Noncurrent	[2],[3]	62,227	63,970	67,999
Decoupling [Domain]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[4]	10,087	13,067	13,235
Regulatory Assets, Noncurrent	[4]	2,803	5,860	2,641
Other Regulatory [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[5]	12,876	11,161	13,902
Regulatory Assets, Noncurrent	[5]	12,085	14,130	9,789
Pension Balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[6]	53,105	50,863	46,247
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		36,591	38,670	40,106
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		179,586	183,035	180,909
Asset Recoverable Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current		13,741	8,054	22,098
Regulatory Liability, Noncurrent		4,740	1,021	9,221
Unrealized Loss On Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current	[1]	2,870	16,624	1,960
Regulatory Liability, Noncurrent	[1]	46	3,265	452
Other Regulatory [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current	[5]	16,600	15,612	11,538
Regulatory Liability, Noncurrent	[5]	7,187	3,926	6,088
Removal Costs [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Noncurrent	[7]	$ 345,614	$ 341,107	$ 331,000

[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] 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, 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 recoverable through utility rates, subject to the aforementioned earnings test. See Note 13.

[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.

[7] Estimated costs of removal on certain regulated properties are collected through rates.