Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,673	$ 1,516		$ 4,988	$ 4,549
Interest cost	4,071	4,662		12,093	13,985
Expected return on plan assets	(5,358)	(5,207)		(16,350)	(15,621)
Amortization of net actuarial loss	0	1		0	5
Defined Benefit Plan, Amortization of Gain (Loss)	5,082	3,604		14,638	10,810
Net periodic benefit cost	5,468	4,576		15,369	13,728
Amount allocated to construction	(695)	(608)		(2,043)	(1,790)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	4,773	3,968		13,326	11,938
Regulatory Pension Disallowance	0	0	$ 10,500	0	10,500
Amortization of Regulatory Balancing Account	675	675	$ 12,500	4,757	14,467
Net amount charged to expense	5,448	4,643		18,083	36,905
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	67	67		195	202
Interest cost	229	281		675	845
Expected return on plan assets	0	0		0	0
Amortization of net actuarial loss	(119)	(117)		(353)	(351)
Defined Benefit Plan, Amortization of Gain (Loss)	173	97		458	290
Net periodic benefit cost	350	328		975	986
Amount allocated to construction	(25)	(25)		(71)	(72)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	325	303		904	914
Regulatory Pension Disallowance	0	0		0	0
Amortization of Regulatory Balancing Account	0	0		0	0
Net amount charged to expense	$ 325	$ 303		$ 904	$ 914