Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Accumulated Other Comprehensive loss, beginning of period				$ (7,162)
Accumulated Other Comprehensive loss, end of period		$ (7,570)	$ (9,081)	(7,570)	$ (9,081)
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Contribution Plan, Cost Recognized				3,600	2,900
Pension Plan [Member]
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Defined Benefit Plan, Service Cost		1,978	2,308	5,866	6,926
Defined Benefit Plan, Interest Cost		4,607	4,597	13,755	13,787
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost		1,978	2,308	5,866	6,926
Defined Benefit Plan, Interest Cost		4,607	4,597	13,755	13,787
Defined Benefit Plan, Expected Return on Plan Assets		(5,017)	(5,174)	(15,051)	(15,522)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		3,555	4,561	10,559	13,683
Defined Benefit Plan, Amortization of loss		57	57	173	173
Defined Benefit Plan, Settlements, Benefit Obligation	[1]	193	0	193	0
Defined Benefit Plan Net Periodic Benefit Cost		5,373	6,349	15,495	19,047
Defined Benefit Plan Amount Allocated To Construction		(1,556)	(2,061)	(4,678)	(5,765)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[2]	(1,542)	(2,171)	(4,762)	(6,511)
Defined Benefit Plan Net Amount Charged To Expense		2,275	2,117	6,055	6,771
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Defined Benefit Plan, Service Cost		119	145	361	435
Defined Benefit Plan, Interest Cost		301	291	901	874
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost		119	145	361	435
Defined Benefit Plan, Interest Cost		301	291	901	874
Defined Benefit Plan, Expected Return on Plan Assets		0	0	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		192	125	575	376
Defined Benefit Plan, Amortization of loss		(117)	50	(351)	148
Defined Benefit Plan, Settlements, Benefit Obligation	[1]	0	0	0	0
Defined Benefit Plan Net Periodic Benefit Cost		495	611	1,486	1,833
Defined Benefit Plan Amount Allocated To Construction		(163)	(218)	(491)	(607)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[2]	0	0	0	0
Defined Benefit Plan Net Amount Charged To Expense		332	393	995	1,226
Qualified Pension Plans, Defined Benefit [Member]
Pension and Other Postretirement Benefit Contributions [Abstract]
Qualified Defined Benefit Pension Plan Contributions				11,300
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year				3,200
Non-Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax		(1,795)	0	(1,795)	0
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Accumulated Other Comprehensive loss, beginning of period		(6,825)	(9,413)	(7,162)	(10,076)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax		371	549	962	1,645
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax		(1,231)	549	(640)	1,645
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax, Portion Attributable to Parent		486	(217)	232	(650)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax		(745)	332	(408)	995
Accumulated Other Comprehensive loss, end of period		(7,570)	(9,081)	(7,570)	(9,081)
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Settlements, Benefit Obligation		$ 193	$ 0	$ 193	$ 0

[1] During the three months ended September 30, 2016, a participant within the Company's Supplemental Executive Retirement Plan elected to have their benefit paid out in a one-time lump-sum cash payment. Accordingly, this transaction qualified for settlement accounting and a pro rata portion of the associated loss in Accumulated Other Comprehensive Loss was immediately recognized in earnings.

[2] 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. See Note 2 in the 2015 Form 10-K.