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	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net prior service cost	$ (200)	$ (200)	$ (400)	$ (400)
Amortization of net loss	3,121	3,755	6,221	7,416
Adjustment due to effects of regulation	(2,642)	(3,352)	(5,283)	(6,610)
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, before tax	279	203	538	406
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, tax	(59)	(42)	(113)	(85)
Other comprehensive (income) loss, defined benefit plan, reclassification adjustment from AOCI, after tax	$ 220	$ 161	$ 425	$ 321