Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,658	$ 1,516		$ 3,315	$ 3,033
Interest cost	4,011	4,661		8,022	9,323
Expected return on plan assets	(5,496)	(5,207)		(10,992)	(10,414)
Amortization of net actuarial loss	0	2		0	4
Defined Benefit Plan, Amortization of Gain (Loss)	4,778	3,603		9,556	7,206
Net periodic benefit cost	4,951	4,575		9,901	9,152
Amount allocated to construction	(680)	(596)		(1,348)	(1,182)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	4,271	3,979		8,553	7,970
Regulatory Pension Disallowance	0	0	$ 10,500	0	10,500
Amortization of Regulatory Balancing Account	1,281	1,281	$ 12,500	4,082	13,792
Net amount charged to expense	5,552	5,260		12,635	32,262
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	64	67		128	135
Interest cost	223	283		446	564
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)	142	96		285	193
Net periodic benefit cost	312	329		625	658
Amount allocated to construction	(23)	(23)		(46)	(47)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	289	306		579	611
Regulatory Pension Disallowance	0	0		0	0
Amortization of Regulatory Balancing Account	0	0		0	0
Net amount charged to expense	$ 289	$ 306		$ 579	$ 611