Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 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		$ (6,825)	$ (9,413)	(6,825)	$ (9,413)
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Contribution Plan, Cost Recognized				2,500	2,000
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		1,944	2,309	3,888	4,618
Defined Benefit Plan, Interest Cost		4,574	4,595	9,148	9,190
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost		1,944	2,309	3,888	4,618
Defined Benefit Plan, Interest Cost		4,574	4,595	9,148	9,190
Defined Benefit Plan, Expected Return on Plan Assets		(5,017)	(5,174)	(10,034)	(10,348)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		3,502	4,561	7,004	9,122
Defined Benefit Plan, Amortization of loss		58	58	116	116
Defined Benefit Plan Net Periodic Benefit Cost		5,061	6,349	10,122	12,698
Defined Benefit Plan Amount Allocated To Construction		(1,574)	(1,879)	(3,122)	(3,704)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[1]	(1,593)	(2,165)	(3,220)	(4,340)
Defined Benefit Plan Net Amount Charged To Expense		1,894	2,305	3,780	4,654
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		121	145	242	290
Defined Benefit Plan, Interest Cost		300	292	600	583
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost		121	145	242	290
Defined Benefit Plan, Interest Cost		300	292	600	583
Defined Benefit Plan, Expected Return on Plan Assets		0	0	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)		192	125	384	251
Defined Benefit Plan, Amortization of loss		(117)	49	(234)	98
Defined Benefit Plan Net Periodic Benefit Cost		496	611	992	1,222
Defined Benefit Plan Amount Allocated To Construction		(164)	(198)	(328)	(389)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	[1]	0	0	0	0
Defined Benefit Plan Net Amount Charged To Expense		332	413	664	833
Qualified Pension Plans, Defined Benefit [Member]
Pension and Other Postretirement Benefit Contributions [Abstract]
Qualified Defined Benefit Pension Plan Contributions				6,100
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year				8,400
Non-Qualified Pension Plans, Defined Benefit [Member]
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		(6,968)	(9,744)	(7,162)	(10,076)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax		269	548	590	1,096
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax		269	548	590	1,096
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax, Portion Attributable to Parent		(126)	(217)	(253)	(433)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax		143	331	337	663
Accumulated Other Comprehensive loss, end of period		$ (6,825)	$ (9,413)	$ (6,825)	$ (9,413)

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