Summary of Significant Accounting Policies Regulatory Asset Disclosure (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory Assets, Current		$ 42,362	$ 69,178
Regulatory Assets, Noncurrent		$ 357,530	370,711
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]	$ 1,315	22,092
Regulatory Assets, Noncurrent	[1]	913	3,447
Asset Recoverable Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current		6,830	8,717
Regulatory Assets, Noncurrent		89	1,949
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[2]	9,989	9,270
Regulatory Assets, Noncurrent	[2]	63,970	76,584
Decoupling [Domain]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[3]	13,067	18,775
Regulatory Assets, Noncurrent	[3]	5,860	6,349
Other Regulatory [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[4]	11,161	10,324
Regulatory Assets, Noncurrent	[4]	14,130	11,362
Pension Balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[5]	50,863	43,748
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		38,670	43,049
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		$ 183,035	$ 184,223

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

[4] 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.

[5] 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.