Income Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Components of Deferred Tax Assets and Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	$ 408,342	$ 386,732
Deferred Tax Liabilities, Regulatory Assets	47,427	51,805
Deferred Tax Liabilities, Regulatory Assets and Liabilities	46,400	55,776
Deferred Tax Liabilities Non Regulated Liabilities	49,683	48,683
Deferred Tax Liabilities, Net	551,852	542,996
Deferred Tax Assets, Tax Deferred Expense, Pension and Postretirement Obligations	4,666	6,537
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	16,699	16,788
Deferred Tax Assets, Tax Credit Carryforwards, Other	514	12,657
Deferred Tax Assets, Net	21,879	35,982
Deferred Tax Assets (Liabilities), Net	529,973	507,014
Accumulated Deferred Investment Tax Credit	48	166
Deferred Tax Liabilities, Other	530,021	507,180
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	31,310	35,117	$ 35,785
Income Tax Reconciliation, State and Local Income Taxes, net of federal tax benefit	4,195	4,666	4,674
Income Tax Reconciliation, Tax Credits, Investment	(118)	(201)	(271)
Income Tax Reconciliation, Differences Regulatory Commission	2,357	2,357	2,357
Income Tax Reconciliation, Gains on company and trust-owned life insurance	(766)	(689)	(864)
Income Tax Reconciliation, Other Adjustments	(1,225)	393	24
Income Tax Expense (Benefit)	$ 35,753	$ 41,643	$ 41,705
Effective Income Tax Rate	40.00%	41.50%	40.80%
Income Tax Disclosure [Abstract]
Operating Loss Carryforwards	$ 3,900
Operating Loss Carryforwards, Expiration Date	Jan. 01, 2028
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	$ 16,699	$ 16,788
Deferred Tax Assets, Tax Credit Carryforwards, General Business	300
Deferred Tax Assets, Charitable Contribution Carryforwards	2,300
Income Tax expense decrease after deferred depletion is realized	900
Tax Provision Current And Deferred [Abstract]
Income tax charge from increased effective Oregon tax rate		600
Current Federal Tax Expense (Benefit)	10,558	14,823	$ (62)
Current State and Local Tax Expense (Benefit)	61	24	(11)
Current Income Tax Expense (Benefit)	10,619	14,847	(73)
Deferred Federal Income Tax Expense (Benefit)	18,729	18,635	35,109
Deferred State and Local Income Tax Expense (Benefit)	6,405	8,161	6,669
Deferred Income Tax Expense (Benefit)	25,134	26,796	41,778
Income Tax Expense (Benefit)	35,753	41,643	41,705
Tax Provision Regulated And Nonutility [Abstract]
Regulated Current Tax Expense Benefit	15,890	24,317	(73)
Regulated Deferred And Local Tax Expense Benefit	20,834	19,518	38,073
Regulated Investments And Credits Tax Expense Benefit	(118)	(201)	(271)
Total Tax Provision Related To Regulated Utility	36,606	43,634	37,729
Non Utility Current Tax Expense Benefit	(5,271)	(9,470)	0
Non Utility Deferred And Local Tax Expense Benefit	4,418	7,479	3,976
Total Tax Provision Related To Non Utility	(853)	(1,991)	3,976
Income Tax Expense (Benefit)	$ 35,753	$ 41,643	$ 41,705