Employee Benefit Plans Employee Benefit Plans 3 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Amounts recognized on the balance sheets:
Non-current liabilities				$ 68,469	$ 57,636
Pension Benefits
Amounts recognized on the balance sheets:
Non-current assets				0	0
Current liabilities				0	0
Non-current liabilities				39,375	33,553
Net amount recognized				(39,375)	(33,553)
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)				3,191	3,626
Net (gain) loss				(37,309)	(31,587)
Regulatory assets (liabilities)	$ 35,213	$ 32,761	$ 32,761	40,500	35,213
Unfunded accrued pension cost				(1,125)	(1,660)
Net liability (asset) recognized				39,375	33,553
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	35,213	32,761
Net loss (gain)	(7,140)	(81)
Amortization of prior service (cost) credit	(434)	0
Amortization of net gain (loss)	(1,419)	(1,255)
Total change in regulatory asset	5,287	2,452
Regulatory asset (liability) at end of year	40,500	35,213	32,761
Defined Benefit Plan Amounts Recognized in Regulatory Asset, Liability New Prior Service Cost (Credit)				0	3,626
Net periodic pension costs	4,447	3,070	4,121
Change in regulatory asset	5,287	2,452
Total recognized in net periodic pension cost and regulatory asset (liability)	9,734	5,522
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service (cost) credit				(434)	(434)
Net gain (loss)	(1,768)	(1,435)
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation				231,852	196,082
Accumulated benefit obligation				215,996	183,036
Fair value of plan assets				$ 192,477	$ 162,529
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate				3.43%	4.43%
Post-Retirement Medical Benefits
Amounts recognized on the balance sheets:
Non-current assets				$ 3,876	$ 2,124
Current liabilities				0	0
Non-current liabilities				0	0
Net amount recognized				3,876	2,124
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)				0	0
Net (gain) loss				5,432	4,459
Regulatory assets (liabilities)	(4,459)	(4,459)	(5,650)	(5,432)	(4,459)
Unfunded accrued pension cost				1,556	2,335
Net liability (asset) recognized				(3,876)	(2,124)
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	(4,459)	(5,650)
Net loss (gain)	1,775	(421)
Amortization of prior service (cost) credit	0	0
Amortization of net gain (loss)	802	770
Total change in regulatory asset	(973)	1,191
Regulatory asset (liability) at end of year	(5,432)	(4,459)	(5,650)
Net periodic pension costs	(779)	(752)	$ (690)
Change in regulatory asset	(973)	1,191
Total recognized in net periodic pension cost and regulatory asset (liability)	(1,752)	439
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service (cost) credit				0	0
Net gain (loss)	$ 796	$ 598
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation				7,395	7,886
Fair value of plan assets				$ 11,271	$ 10,010
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate				3.12%	4.20%
Minimum | Pension Benefits
Weighted-average assumptions used to determine benefit obligations at December 31:
Rate of compensation increase				3.00%	3.00%
Maximum | Pension Benefits
Weighted-average assumptions used to determine benefit obligations at December 31:
Rate of compensation increase				8.00%	8.00%