Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,517	$ 1,807
Interest cost	4,662	4,183
Expected return on plan assets	(5,207)	(5,151)
Amortization of net actuarial loss	2	11
Defined Benefit Plan, Amortization of Gain (Loss)	3,603	4,523
Net periodic benefit cost	4,577	5,373
Amount allocated to construction	(586)	(682)
Amount deferred to regulatory balancing account(1)	0	(2,756)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	3,991	1,935
Regulatory Pension Disallowance	10,500	0
Amortization of Regulatory Balancing Account	12,511	0
Net amount charged to expense	27,002	1,935
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	68	80
Interest cost	281	241
Expected return on plan assets	0	0
Amortization of net actuarial loss	(117)	(117)
Defined Benefit Plan, Amortization of Gain (Loss)	97	110
Net periodic benefit cost	329	314
Amount allocated to construction	(24)	(27)
Amount deferred to regulatory balancing account(1)	0	0
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	305	287
Regulatory Pension Disallowance	0	0
Amortization of Regulatory Balancing Account	0	0
Net amount charged to expense	$ 305	$ 287