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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net prior service cost	$ (200)	$ (200)	$ (600)	$ (600)
Amortization of net loss	2,369	3,120	8,299	9,341
Adjustment due to effects of regulation	(1,784)	(2,642)	(6,525)	(7,925)
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, before tax	385	278	1,174	816
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, tax	(81)	(58)	(247)	(171)
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, after tax	$ 304	$ 220	$ 927	$ 645