Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (9,744)	$ (6,193)	$ (10,076)	$ (6,358)
Amortization of prior service costs		0	(2)	0	(4)
Amortization of actuarial losses		548	276	1,096	551
Total reclassifications before tax		548	274	1,096	547
Tax expense		(217)	(108)	(433)	(216)
Total reclassifications for the period		331	166	663	331
Ending balance		(9,413)	(6,027)	(9,413)	(6,027)
General Discussion of Pension and Other Postretirement Benefits [Abstract]
Qualified Defined Benefit Pension Plan Contributions				5,800
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year				9,200
Defined Contribution Plan, Cost Recognized				2,000	1,900
Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost		2,309	1,918	4,618	3,836
Defined Benefit Plan, Interest Cost		4,595	4,512	9,190	9,024
Defined Benefit Plan Expected Return On Plan Assets		(5,174)	(4,886)	(10,348)	(9,772)
Defined Benefit Plan Amortization Of Net Actuarial Loss		4,561	2,580	9,122	5,160
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		58	56	116	112
Defined Benefit Plan Net Periodic Benefit Cost		6,349	4,180	12,698	8,360
Defined Benefit Plan Amount Allocated To Construction		(1,879)	(1,201)	(3,704)	(2,402)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[1]	(2,165)	(1,123)	(4,340)	(2,224)
Defined Benefit Plan Net Amount Charged To Expense		2,305	1,856	4,654	3,734
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost		145	136	290	271
Defined Benefit Plan, Interest Cost		292	309	583	619
Defined Benefit Plan Expected Return On Plan Assets		0	0	0	0
Defined Benefit Plan Amortization Of Net Actuarial Loss		125	46	251	92
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		49	49	98	98
Defined Benefit Plan Net Periodic Benefit Cost		611	540	1,222	1,080
Defined Benefit Plan Amount Allocated To Construction		(198)	(171)	(389)	(341)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[1]	0	0	0	0
Defined Benefit Plan Net Amount Charged To Expense		$ 413	$ 369	$ 833	$ 739

[1] 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 the interest recognized when amounts are collected in rates.