Accumulated Other Comprehensive Loss - Reclassification out of Accumulated Other Comprehensive Income (Details) - Accumulated Defined Benefit and Postretirement Benefit Plans Adjustment Attributable to Parent [Member] - Reclassification out of Accumulated Other Comprehensive Income [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net prior service cost	$ (200)	$ (200)	$ (400)	$ (400)
Amortization of net loss	2,739	3,121	5,930	6,221
Adjustment due to effects of regulation	(2,149)	(2,642)	(4,741)	(5,283)
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, before tax	390	279	789	538
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, tax	(82)	(59)	(166)	(113)
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, after tax	$ 308	$ 220	$ 623	$ 425