Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 22, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Amounts That Will Be Amortized From Accumulated Other Comprehensive Income Loss In Next Fiscal Year Regulatory	$ 17,300
Defined Benefit Plan Amortization Of Net Gains Losses Regulatory	17,700
Defined Benefit Plan Amortization Of Net Prior Service Cost Credit Regulatory	$ 400
Defined Benefit Plan, Health Care Cost Trend Rate For Pre-65 Population Assumed for Next Fiscal Year	7.50%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate, Decrease	4.75%
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate				2026
Defined Contribution Plan, Cost Recognized	$ 5,400	$ 4,600	$ 3,700
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	(6,951)
Accumulated Other Comprehensive loss, end of period	(8,438)	(6,951)
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Amortization of loss	800
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	44
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(39)
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	478
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(428)
Pension and Other Postretirement Benefit Contributions [Abstract]
Qualified Defined Benefit Pension Plan Contributions	$ 19,400
Fixed Income Pension Assets				$ 2,781
Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Net Periodic Benefit Costs Overhead Charge Percentage	25.00%
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Net Periodic Benefit Costs Overhead Charge Percentage	35.00%
Domestic Large Cap Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	29.30%
Domestic Small/Mid Cap Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	6.90%
Foreign Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	28.00%
Emerging Market Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	11.80%
Government Or Credit Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	17.50%
High Yield Bond Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	2.00%
Emerging Market Debt [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	3.50%
Real Estate Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	1.00%
Pension Costs [Member]
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Regulatory Assets Prior Service Cost				49	$ 176
Regulatory Assets Actuarial Loss				175,035	177,660
Regulatory Assets				175,084	177,836
Postretirement Benefit Costs [Member]
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Regulatory Assets Prior Service Cost				(2,206)	(2,675)
Regulatory Assets Actuarial Loss				6,964	7,874
Regulatory Assets				4,758	5,199
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				287,895	252,115
Defined Benefit Plan, Benefit Obligation	$ 457,839	445,628	445,628	486,289	457,839	$ 445,628
Defined Benefit Plan, Fair Value of Plan Assets	257,714	249,338	249,338	287,925	257,714	$ 249,338
Defined Benefit Plan, Funded Status of Plan				$ (198,364)	$ (200,125)
Reconciliation of change in benefit obligation
Obligation at beginning of period	457,839	445,628
Defined Benefit Plan, Service Cost	7,090	7,083	8,267
Defined Benefit Plan, Interest Cost	18,111	18,399	18,360
Defined Benefit Plan, Actuarial Loss	34,829	7,688
Defined Benefit Plan, Benefits Paid	31,580	20,959	35,923
Obligation and end of period	486,289	457,839	445,628
Reconciliation of change in plan assets
Fair value of plan assets, beginning of period	257,714	249,338
Defined Benefit Plan, Actual Return on Plan Assets	40,308	12,593
Defined Benefit Plan, Contributions by Employer	21,483	16,742
Defined Benefit Plan, Benefits Paid	31,580	20,959	35,923
Fair value of plan assets, end of period	287,925	257,714	249,338
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Regulatory assets defined benefit pension actuarial (gain) loss	12,177	14,005	419
Regulatory assets, Adjustment for Defined Benefit Plan Prior Service Cost	(127)	(230)	(230)
Regulatory asset, Adjustment for Defined Benefit Plan Amortization of Actuarial Loss	(14,802)	(13,238)	(16,372)
Regulatory Asset Adjustment Defined Benefit Plan	(2,752)	537	(16,183)
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost	7,090	7,083	8,267
Defined Benefit Plan, Interest Cost	18,111	18,399	18,360
Defined Benefit Plan, Expected Return on Plan Assets	(20,433)	(20,054)	(20,676)
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	127	231	231
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	15,748	14,624	18,609
Defined Benefit Plan, Amortization of loss	0	193	0
Defined Benefit Plan Net Periodic Benefit Cost	20,643	20,476	24,791
Defined Benefit Plan Amount Allocated To Construction	(6,597)	(5,746)	(6,834)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	(6,542)	(6,252)	(8,241)
Defined Benefit Plan Net Amount Charged To Expense	$ 7,504	$ 8,478	$ 9,716
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	3.99%	4.17%	3.82%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.50%	7.50%	7.50%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate				3.52%	4.00%	4.21%
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation Expected Long Term Return On Assets				7.50%	7.50%	7.50%
Pension and Other Postretirement Benefit Contributions [Abstract]
Qualified Defined Benefit Pension Plan Contributions	$ 21,483	$ 16,742
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	17,710
Defined Benefit Plan, Benefits Paid	31,580	20,959	$ 35,923
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months				$ 22,679
Defined Benefit Plan, Expected Future Benefit Payments, Year Two				23,546
Defined Benefit Plan, Expected Future Benefit Payments, Year Three				24,542
Defined Benefit Plan, Expected Future Benefit Payments, Year Four				25,471
Defined Benefit Plan, Expected Future Benefit Payments, Year Five				26,095
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter				$ 145,065
Defined Benefit Plan, Settlements, Benefit Obligation	$ 0	$ 0	$ 0
Pension Plan [Member] | Minimum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Rate Of Compensation Increase	3.25%	3.25%	3.25%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation Rate Of Compensation Increase				3.25%	3.25%	3.25%
Pension Plan [Member] | Maximum [Member]
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Rate Of Compensation Increase	4.50%	4.50%	5.00%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation Rate Of Compensation Increase				4.50%	4.50%	4.50%
Pension Plan [Member] | Domestic Large Cap Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				$ 102,851	$ 55,496
Pension Plan [Member] | Domestic Small/Mid Cap Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				16,423	28,861
Pension Plan [Member] | Foreign Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				77,286	47,750
Pension Plan [Member] | Emerging Market Equity Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				28,743	13,457
Pension Plan [Member] | Fixed Income Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables					6,719
Pension Plan [Member] | Government Or Credit Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				33,081	52,915
Pension Plan [Member] | High Yield Bond Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				2,777	14,072
Pension Plan [Member] | Emerging Market Debt [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				12,605	8,504
Pension Plan [Member] | Real Estate Funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				5,544	18,739
Pension Plan [Member] | Absolute Return Strategy Investment Securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				189
Pension Plan [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total Pension Asset Investments Net Of Receivables And Payables				5,615	2,491
Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Benefit Obligation	$ 29,395	$ 31,049	$ 31,049	28,927	29,395	$ 31,049
Defined Benefit Plan, Fair Value of Plan Assets	0	0	0	0	0	$ 0
Defined Benefit Plan, Funded Status of Plan				$ (28,927)	$ (29,395)
Reconciliation of change in benefit obligation
Obligation at beginning of period	29,395	31,049
Defined Benefit Plan, Service Cost	341	391	527
Defined Benefit Plan, Interest Cost	1,141	1,175	1,179
Defined Benefit Plan, Actuarial Loss	(213)	(1,488)
Defined Benefit Plan, Benefits Paid	1,737	1,732	2,018
Obligation and end of period	28,927	29,395	31,049
Reconciliation of change in plan assets
Fair value of plan assets, beginning of period	0	0
Defined Benefit Plan, Actual Return on Plan Assets	0	0
Defined Benefit Plan, Contributions by Employer	1,737	1,732
Defined Benefit Plan, Benefits Paid	1,737	1,732	2,018
Fair value of plan assets, end of period	0	0	0
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Regulatory assets defined benefit pension actuarial (gain) loss	(214)	(1,488)	2,724
Regulatory assets, Adjustment for Defined Benefit Plan Prior Service Cost	468	468	(197)
Regulatory asset, Adjustment for Defined Benefit Plan Amortization of Actuarial Loss	(696)	(705)	(554)
Regulatory Asset Adjustment Defined Benefit Plan	(442)	(1,725)	1,973
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost	341	391	527
Defined Benefit Plan, Interest Cost	1,141	1,175	1,179
Defined Benefit Plan, Expected Return on Plan Assets	0	0	0
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	(468)	(468)	197
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	696	705	554
Defined Benefit Plan, Amortization of loss	0	0	0
Defined Benefit Plan Net Periodic Benefit Cost	1,710	1,803	2,457
Defined Benefit Plan Amount Allocated To Construction	(587)	(600)	(808)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	0	0	0
Defined Benefit Plan Net Amount Charged To Expense	$ 1,123	$ 1,203	$ 1,649
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	3.85%	4.00%	3.74%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate				3.44%	3.85%	4.00%
Pension and Other Postretirement Benefit Contributions [Abstract]
Qualified Defined Benefit Pension Plan Contributions	$ 1,737	$ 1,732
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	1,835
Defined Benefit Plan, Benefits Paid	1,737	1,732	$ 2,018
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months				$ 1,835
Defined Benefit Plan, Expected Future Benefit Payments, Year Two				1,871
Defined Benefit Plan, Expected Future Benefit Payments, Year Three				1,861
Defined Benefit Plan, Expected Future Benefit Payments, Year Four				1,904
Defined Benefit Plan, Expected Future Benefit Payments, Year Five				1,886
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter				9,261
Defined Benefit Plan, Settlements, Benefit Obligation	0	0	0
Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Benefit Obligation	423,500	423,500		449,700	$ 423,500
Defined Benefit Plan, Fair Value of Plan Assets	257,700	257,700		287,900	257,700
Defined Benefit Plan, Funded Status of Plan				(161,700)
Reconciliation of change in benefit obligation
Obligation at beginning of period	423,500
Obligation and end of period	449,700	423,500
Reconciliation of change in plan assets
Fair value of plan assets, beginning of period	257,700
Fair value of plan assets, end of period	287,900	257,700
Defined Benefit Plan Amounts Recognized In Regulatory Amortization And Other Comprehensive Income Net Prior Service Cost Credit Before Tax [Abstract]
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax				0	1
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				13,266	11,434
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax				13,266	11,435
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	2,777	(1,196)	(2,549)
Other Comprehensive Income (Loss), Amortization, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost Recognized in Net Periodic Benefit Cost, before Tax	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax	(946)	1,386	(2,236)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax	1,831	383	(4,785)
Pension and Other Postretirement Benefit Contributions [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	15,500
Defined Benefit Plan, Settlements, Benefit Obligation	0	193	0
Non-Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Benefit Obligation	34,300	34,300		36,600	$ 34,300
Defined Benefit Plan, Fair Value of Plan Assets	0			0
Reconciliation of change in benefit obligation
Obligation at beginning of period	34,300
Obligation and end of period	36,600	34,300
Reconciliation of change in plan assets
Fair value of plan assets, end of period	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	(6,951)	(7,162)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	(2,794)	(1,196)
Other Comprehensive Income (Loss), Amortization, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost Recognized in Net Periodic Benefit Cost, before Tax	946	1,386
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax	0	193
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax	(1,848)	383
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax, Portion Attributable to Parent	361	(172)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	(1,487)	211
Accumulated Other Comprehensive loss, end of period	(8,438)	(6,951)	(7,162)
Western States Office and Professional Employees Union Pension Fund [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer Plans, Withdrawal Obligation				$ 7,100			$ 8,300
Multiemployer Plans, Plan Contributions	600
Multiemployer Plan, Period Contributions	$ 600	$ 600	$ 600
Western States Office and Professional Employees Union Pension Fund [Member] | Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer Plans, Collective-Bargaining Arrangement, Percentage of Contributions				4.00%
Western States Office and Professional Employees Union Pension Fund [Member] | Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer Plans, Collective-Bargaining Arrangement, Percentage of Contributions				5.00%