Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,657	$ 1,517
Interest cost	4,011	4,662
Expected return on plan assets	(5,496)	(5,207)
Amortization of net actuarial loss	0	2
Defined Benefit Plan, Amortization of Gain (Loss)	4,778	3,603
Net periodic benefit cost	4,950	4,577
Amount allocated to construction	(668)	(586)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	4,282	3,991
Regulatory Pension Disallowance	0	10,500
Amortization of Regulatory Balancing Account	2,801	12,511
Net amount charged to expense	7,083	27,002
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	64	68
Interest cost	223	281
Expected return on plan assets	0	0
Amortization of net actuarial loss	(117)	(117)
Defined Benefit Plan, Amortization of Gain (Loss)	143	97
Net periodic benefit cost	313	329
Amount allocated to construction	(23)	(24)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	290	305
Regulatory Pension Disallowance	0	0
Amortization of Regulatory Balancing Account	0	0
Net amount charged to expense	$ 290	$ 305