Employee Benefit Plans Employee Benefit Plans 3 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Amounts recognized on the balance sheets:
Non-current liabilities				$ 57,636	$ 57,695
Pension Benefits
Amounts recognized on the balance sheets:
Non-current assets				0	0
Current liabilities				0	0
Non-current liabilities				33,553	34,042
Net amount recognized				(33,553)	(34,042)
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)				3,626	0
Net (gain) loss				(31,587)	(32,761)
Regulatory assets (liabilities)	$ 32,761	$ 25,828	$ 25,828	35,213	32,761
Unfunded accrued pension cost				1,660	1,281
Net liability (asset) recognized				33,553	34,042
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	32,761	25,828
Net loss (gain)	(81)	(7,856)
Amortization of net gain (loss)	(1,255)	(923)
Total change in regulatory asset	2,452	6,933
Regulatory asset (liability) at end of year	35,213	32,761	25,828
Net periodic pension costs	3,070	4,121	4,126
Change in regulatory asset	2,452	6,933
Total recognized in net periodic pension cost and regulatory asset (liability)	5,522	11,054
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service (cost) credit				(434)	0
Net gain (loss)	(1,435)	(1,378)
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation				196,082	207,690
Accumulated benefit obligation				183,036	190,438
Fair value of plan assets				$ 162,529	$ 173,648
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate				4.43%	3.76%
Post-Retirement Medical Benefits
Amounts recognized on the balance sheets:
Non-current assets				$ 2,124	$ 2,562
Current liabilities				0	0
Non-current liabilities				0	0
Net amount recognized				2,124	2,562
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)				0	0
Net (gain) loss				4,459	5,650
Regulatory assets (liabilities)	(5,650)	(5,515)	(5,515)	(4,459)	(5,650)
Unfunded accrued pension cost				2,335	3,088
Net liability (asset) recognized				(2,124)	(2,562)
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	(5,650)	(5,515)
Net loss (gain)	(421)	910
Amortization of net gain (loss)	770	775
Total change in regulatory asset	1,191	(135)
Regulatory asset (liability) at end of year	(4,459)	(5,650)	(5,515)
Net periodic pension costs	(752)	(690)	$ (606)
Change in regulatory asset	1,191	(135)
Total recognized in net periodic pension cost and regulatory asset (liability)	439	(825)
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service (cost) credit				0	0
Net gain (loss)	$ 598	$ 727
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation				7,886	8,491
Fair value of plan assets				$ 10,010	$ 11,053
Weighted-average assumptions used to determine benefit obligations at December 31:
Discount rate				4.20%	3.52%
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%