Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,516		$ 1,757	$ 4,549	$ 5,371
Interest cost	4,662		4,336	13,985	12,702
Expected return on plan assets	(5,207)		(5,143)	(15,621)	(15,444)
Amortization of net actuarial loss	1		11	5	32
Defined Benefit Plan, Amortization of Gain (Loss)	3,604		5,650	10,810	14,697
Net periodic benefit cost	4,576		6,611	13,728	17,358
Amount allocated to construction	(608)		(659)	(1,790)	(2,026)
Amount deferred to regulatory balancing account(1)	0		(3,878)	0	(9,381)
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	3,968		2,074	11,938	5,951
Regulatory Pension Disallowance	0		0	10,500	0
Amortization of Regulatory Balancing Account	675	$ 12,500	0	14,467	0
Net amount charged to expense	4,643		2,074	36,905	5,951
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	67		80	202	239
Interest cost	281		241	845	723
Expected return on plan assets	0		0	0	0
Amortization of net actuarial loss	(117)		(117)	(351)	(351)
Defined Benefit Plan, Amortization of Gain (Loss)	97		110	290	332
Net periodic benefit cost	328		314	986	943
Amount allocated to construction	(25)		(27)	(72)	(82)
Amount deferred to regulatory balancing account(1)	0		0	0	0
Defined Benefit Plan Net Periodic Benefit Cost Charged To Expense	303		287	914	861
Regulatory Pension Disallowance	0		0	0	0
Amortization of Regulatory Balancing Account	0		0	0	0
Net amount charged to expense	$ 303		$ 287	$ 914	$ 861
Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Net Periodic Benefit Costs Overhead Charge Percentage				25.00%
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Net Periodic Benefit Costs Overhead Charge Percentage				35.00%