Employee Benefit Plans Employee Benefit Plans 4 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets		4.20%
Expected long-term return on plan assets		4.20%
Pension Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 4,441	$ 5,342	$ 4,999
Interest cost	8,527	7,646	7,904
Expected return on plan assets	(10,374)	(11,172)	(9,705)
Amortization of prior service cost (credit)	434	0	0
Amortization of actuarial (gain) loss	1,419	1,254	923
Net periodic pension cost under accounting standards	4,447	3,070	4,121
Regulatory adjustment	593	0	(465)
Total expense recognized, before surcharges and allocation to overhead pool	$ 3,854	$ 3,070	$ 4,586
Weighted-average assumptions used to determine net periodic cost:
Discount rate	4.43%	3.76%	4.44%
Expected long-term return on plan assets	6.50%	6.50%	6.50%
Expected long-term return on plan assets	6.50%	6.50%	6.50%
Post-Retirement Medical Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 186	$ 218	$ 227
Interest cost	285	292	324
Expected return on plan assets	(449)	(493)	(466)
Amortization of prior service cost (credit)	0	0	0
Amortization of actuarial (gain) loss	(801)	(769)	(775)
Net periodic pension cost under accounting standards	(779)	(752)	(690)
Regulatory adjustment	0	0	0
Total expense recognized, before surcharges and allocation to overhead pool	$ (779)	$ (752)	$ (690)
Weighted-average assumptions used to determine net periodic cost:
Discount rate	4.20%	3.52%	3.97%
Union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets	6.00%	6.00%	6.00%
Expected long-term return on plan assets	6.00%	6.00%	6.00%
Non-union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets		4.20%	4.20%
Expected long-term return on plan assets		4.20%	4.20%
Minimum | Pension Benefits
Weighted-average assumptions used to determine net periodic cost:
Rate of compensation increase	3.00%	3.00%
Maximum | Pension Benefits
Weighted-average assumptions used to determine net periodic cost:
Rate of compensation increase	8.00%	8.00%