Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,714	$ 1,658	$ 3,428	$ 3,315
Interest cost	3,342	4,011	6,685	8,022
Expected return on plan assets	(6,099)	(5,496)	(12,198)	(10,992)
Amortization of net actuarial loss	0	0	0	0
Defined Benefit Plan, Amortization of Gain (Loss)	5,500	4,778	11,001	9,556
Net periodic benefit cost	4,457	4,951	8,916	9,901
Amount allocated to construction	(743)	(680)	(1,469)	(1,348)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	3,714	4,271	7,447	8,553
Amortization of Regulatory Balancing Account	1,281	1,281	4,082	4,082
Net amount charged to expense	4,995	5,552	11,529	12,635
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	56	64	111	128
Interest cost	165	223	330	446
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	(117)	(117)	(234)	(234)
Defined Benefit Plan, Amortization of Gain (Loss)	131	142	262	285
Net periodic benefit cost	235	312	469	625
Amount allocated to construction	(22)	(23)	(43)	(46)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	213	289	426	579
Amortization of Regulatory Balancing Account	0	0	0	0
Net amount charged to expense	$ 213	$ 289	$ 426	$ 579