Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,530	$ 1,714
Interest cost	3,659	3,343
Expected return on plan assets	(6,427)	(6,099)
Amortization of net actuarial loss	0	0
Defined Benefit Plan, Amortization of Gain (Loss)	3,198	5,501
Net periodic benefit cost	1,960	4,459
Amount allocated to construction	(664)	(726)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	1,296	3,733
Amortization of Regulatory Balancing Account	2,801	2,801
Net amount charged to expense	4,097	6,534
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	47	55
Interest cost	180	165
Expected return on plan assets	0	0
Amortization of net actuarial loss	(83)	(117)
Defined Benefit Plan, Amortization of Gain (Loss)	99	131
Net periodic benefit cost	243	234
Amount allocated to construction	(18)	(21)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	225	213
Amortization of Regulatory Balancing Account	0	0
Net amount charged to expense	$ 225	$ 213