Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	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	$ 13,800
Defined Benefit Plan Amortization Of Net Gains Losses Regulatory	14,100
Defined Benefit Plan Amortization Of Net Prior Service Cost Credit Regulatory	$ 300
Defined Benefit Plan, Health Care Cost Trend Rate For Pre-65 Population Assumed for Next Fiscal Year	7.00%
Defined Benefit Plan, Health Care Cost Trend Rate For Post-65 Population Assumed for Next Fiscal Year	7.00%
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate, Decrease	4.75%
Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate				2025
Defined Contribution Plan, Cost Recognized	$ 4,600	$ 3,700	$ 3,400
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	(6,951)	(7,162)
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Amortization of loss	900
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	51
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(45)
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	644
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(577)
Pension and Other Postretirement Benefit Contributions [Abstract]
Qualified Defined Benefit Pension Plan Contributions	$ 14,500
Fixed Income Pension Assets				$ 6,719
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	18.00%
Domestic Small/Mid Cap Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	10.00%
Foreign Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	18.00%
Emerging Market Equity Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	5.00%
Government Or Credit Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	20.00%
High Yield Bond Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	5.00%
Emerging Market Debt [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	5.00%
Real Estate Funds [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	7.00%
Absolute Return Strategy Investment Securities [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Defined Benefit Plan Target Plan Asset Allocations	12.00%
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				176	$ 406
Regulatory Assets Actuarial Loss				177,660	176,894
Regulatory Assets				177,836	177,300
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				(2,675)	(3,143)
Regulatory Assets Actuarial Loss				7,874	10,067
Regulatory Assets				5,199	6,924
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				252,115	249,326
Defined Benefit Plan, Benefit Obligation	$ 445,628	487,278	487,278	457,839	445,628	$ 487,278
Defined Benefit Plan, Fair Value of Plan Assets	249,338	279,164	279,164	257,714	249,338	$ 279,164
Defined Benefit Plan, Funded Status of Plan				$ (200,125)	$ (196,290)
Reconciliation of change in benefit obligation
Obligation at beginning of period	445,628	487,278
Defined Benefit Plan, Service Cost	7,083	8,267	7,213
Defined Benefit Plan, Interest Cost	18,399	18,360	18,198
Defined Benefit Plan, Plan Amendments	0	0
Defined Benefit Plan, Actuarial Loss	7,688	(32,354)
Defined Benefit Plan, Benefits Paid	20,959	35,923	19,932
Obligation and end of period	457,839	445,628	487,278
Reconciliation of change in plan assets
Fair value of plan assets, beginning of period	249,338	279,164
Defined Benefit Plan, Actual Return on Plan Assets	12,593	(9,599)
Defined Benefit Plan, Contributions by Employer	16,742	15,696
Defined Benefit Plan, Benefits Paid	20,959	35,923	19,932
Fair value of plan assets, end of period	257,714	249,338	279,164
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	14,005	419	83,027
Regulatory assets, Adjustment for Defined Benefit Plan Prior Service Cost	(230)	(230)	(230)
Regulatory asset, Adjustment for Defined Benefit Plan Amortization of Actuarial Loss	(13,238)	(16,372)	(9,823)
Regulatory Asset Adjustment Defined Benefit Plan	537	(16,183)	72,974
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost	7,083	8,267	7,213
Defined Benefit Plan, Interest Cost	18,399	18,360	18,198
Defined Benefit Plan, Expected Return on Plan Assets	(20,054)	(20,676)	(19,496)
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	231	231	223
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	14,624	18,609	10,914
Defined Benefit Plan, Amortization of loss	193	0	0
Defined Benefit Plan Net Periodic Benefit Cost	20,476	24,791	17,052
Defined Benefit Plan Amount Allocated To Construction	(5,746)	(6,834)	(4,625)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	(6,252)	(8,241)	(4,578)
Defined Benefit Plan Net Amount Charged To Expense	$ 8,478	$ 9,716	$ 7,849
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	4.17%	3.82%	4.71%
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				4.00%	4.21%	3.85%
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	$ 16,742	$ 15,696
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	21,380
Defined Benefit Plan, Benefits Paid	20,959	35,923	$ 19,932
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months				$ 22,171
Defined Benefit Plan, Expected Future Benefit Payments, Year Two				23,088
Defined Benefit Plan, Expected Future Benefit Payments, Year Three				23,953
Defined Benefit Plan, Expected Future Benefit Payments, Year Four				24,782
Defined Benefit Plan, Expected Future Benefit Payments, Year Five				25,690
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter				$ 136,699
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%	5.00%	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%	5.00%
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				$ 55,496	$ 44,528
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				28,861	23,495
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				47,750	43,548
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				13,457	11,120
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					0
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				52,915	48,456
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				14,072	12,298
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				8,504	7,746
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				18,739	17,261
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				3,111	36,758
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				2,491	4,116
Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Benefit Obligation	$ 31,049	$ 32,072	$ 32,072	29,395	31,049	$ 32,072
Defined Benefit Plan, Fair Value of Plan Assets	0	0	0	0	0	$ 0
Defined Benefit Plan, Funded Status of Plan				$ (29,395)	$ (31,049)
Reconciliation of change in benefit obligation
Obligation at beginning of period	31,049	32,072
Defined Benefit Plan, Service Cost	391	527	483
Defined Benefit Plan, Interest Cost	1,175	1,179	1,252
Defined Benefit Plan, Plan Amendments	0	(3,435)
Defined Benefit Plan, Actuarial Loss	(1,488)	2,724
Defined Benefit Plan, Benefits Paid	1,732	2,018	1,871
Obligation and end of period	29,395	31,049	32,072
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	1,732	2,018
Defined Benefit Plan, Benefits Paid	1,732	2,018	1,871
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	(1,488)	2,724	3,454
Regulatory assets, Adjustment for Defined Benefit Plan Prior Service Cost	468	(197)	(197)
Regulatory asset, Adjustment for Defined Benefit Plan Amortization of Actuarial Loss	(705)	(554)	(221)
Regulatory Asset Adjustment Defined Benefit Plan	(1,725)	1,973	3,036
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Defined Benefit Plan, Service Cost	391	527	483
Defined Benefit Plan, Interest Cost	1,175	1,179	1,252
Defined Benefit Plan, Expected Return on Plan Assets	0	0	0
Defined Benefit Plan, Amortization of Transition Obligations (Assets)	(468)	197	197
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	705	554	221
Defined Benefit Plan, Amortization of loss	0	0	0
Defined Benefit Plan Net Periodic Benefit Cost	1,803	2,457	2,153
Defined Benefit Plan Amount Allocated To Construction	(600)	(808)	(702)
Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account	0	0	0
Defined Benefit Plan Net Amount Charged To Expense	$ 1,203	$ 1,649	$ 1,451
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	4.00%	3.74%	4.45%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate				3.85%	4.00%	3.74%
Pension and Other Postretirement Benefit Contributions [Abstract]
Qualified Defined Benefit Pension Plan Contributions	$ 1,732		$ 2,018
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	1,876
Defined Benefit Plan, Benefits Paid	1,732	$ 2,018	1,871
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months				$ 1,876
Defined Benefit Plan, Expected Future Benefit Payments, Year Two				1,893
Defined Benefit Plan, Expected Future Benefit Payments, Year Three				1,977
Defined Benefit Plan, Expected Future Benefit Payments, Year Four				2,020
Defined Benefit Plan, Expected Future Benefit Payments, Year Five				2,054
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter				10,189
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	411,800	411,800		423,500	$ 411,800
Defined Benefit Plan, Fair Value of Plan Assets	249,300	249,300		257,700	249,300
Defined Benefit Plan, Funded Status of Plan				(165,800)
Reconciliation of change in benefit obligation
Obligation at beginning of period	411,800
Obligation and end of period	423,500	411,800
Reconciliation of change in plan assets
Fair value of plan assets, beginning of period	249,300
Fair value of plan assets, end of period	257,700	249,300
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				1	1
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax				11,434	11,870
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax				11,435	11,871
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	(1,196)	(2,549)	7,221
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	7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax	1,386	(2,236)	(1,091)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax	383	(4,785)	6,137
Pension and Other Postretirement Benefit Contributions [Abstract]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	19,400
Defined Benefit Plan, Settlements, Benefit Obligation	193	0	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	33,800	33,800		34,300	$ 33,800
Defined Benefit Plan, Fair Value of Plan Assets	0			0
Reconciliation of change in benefit obligation
Obligation at beginning of period	33,800
Obligation and end of period	34,300	33,800
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	(7,162)	(10,076)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	(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	1,386	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax	193	2,236
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax	383	4,785
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax, Portion Attributable to Parent	(172)	(1,871)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	211	2,914
Accumulated Other Comprehensive loss, end of period	(6,951)	(7,162)	(10,076)
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				$ 7,500			$ 8,300
Multiemployer Plans, Withdrawal Payment Per Year For Next 20 Years	600
Multiemployer Plan, Period Contributions	$ 600	$ 600	$ 400
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%