Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (9,413)	$ (6,027)	$ (10,076)	$ (6,358)
Amortization of prior service costs		0	(1)	0	(5)
Amortization of actuarial losses		549	275	1,645	826
Total reclassifications before tax		549	274	1,645	821
Tax expense		(217)	(108)	(650)	(324)
Total reclassifications for the period		332	166	995	497
Ending balance		(9,081)	(5,861)	(9,081)	(5,861)
General Discussion of Pension and Other Postretirement Benefits [Abstract]
Qualified Defined Benefit Pension Plan Contributions				11,800
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year				2,300
Defined Contribution Plan, Cost Recognized				2,900	2,800
Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Defined Benefit Plan, Service Cost		2,308	1,919	6,926	5,755
Defined Benefit Plan, Interest Cost		4,597	4,511	13,787	13,535
Defined Benefit Plan Expected Return On Plan Assets		(5,174)	(4,887)	(15,522)	(14,659)
Defined Benefit Plan Amortization Of Net Actuarial Loss		4,561	2,579	13,683	7,739
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		57	56	173	168
Defined Benefit Plan Net Periodic Benefit Cost		6,349	4,178	19,047	12,538
Defined Benefit Plan Amount Allocated To Construction		(2,061)	(1,242)	(5,765)	(3,644)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[1]	(2,171)	(1,107)	(6,511)	(3,331)
Defined Benefit Plan Net Amount Charged To Expense		2,117	1,829	6,771	5,563
Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Defined Benefit Plan, Service Cost		145	136	435	407
Defined Benefit Plan, Interest Cost		291	309	874	928
Defined Benefit Plan Expected Return On Plan Assets		0	0	0	0
Defined Benefit Plan Amortization Of Net Actuarial Loss		125	46	376	138
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		50	50	148	148
Defined Benefit Plan Net Periodic Benefit Cost		611	541	1,833	1,621
Defined Benefit Plan Amount Allocated To Construction		(218)	(177)	(607)	(518)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[1]	0	0	0	0
Defined Benefit Plan Net Amount Charged To Expense		$ 393	$ 364	$ 1,226	$ 1,103

[1] The deferral of defined benefit 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. The balancing account includes the expectation of higher net periodic benefit costs than costs recovered in rates in the near-term with lower net periodic benefit costs than costs recovered in rates expected in future years. Deferred pension expense balances include accrued interest at the utility’s authorized rate of return, with the equity portion of the interest recognized when amounts are collected in rates.