Accumulated Other Comprehensive Loss - Reclassification out of Accumulated Other Comprehensive Income (Details) - Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] - Reclassification out of Accumulated Other Comprehensive Income [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net prior service cost	$ (794)	$ (794)	$ (904)
Amortization of net loss	5,586	17,074	(15,554)
Adjustment due to effects of regulation	(10,006)	(19,309)	18,947
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, before tax	(5,214)	(3,029)	2,489
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, tax	1,095	636	(523)
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, after tax	$ (4,119)	$ (2,393)	$ 1,966