Pension and Other Postretirement Benefits Costs - Schedule of Amounts Realized in Other Comprehensive Loss (Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory Asset, Net actuarial loss (gain)	$ 16,170	$ 10,424	$ 14,261
Regulatory Asset, Prior service cost	0	(7)	(42)
Regulatory Asset, Actuarial loss	(18,627)	(14,057)	(18,761)
Regulatory Asset, Total	(2,457)	(3,640)	(4,542)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory Asset, Net actuarial loss (gain)	145	1,809	(327)
Regulatory Asset, Prior service cost	468	468	468
Regulatory Asset, Actuarial loss	(607)	(369)	(448)
Regulatory Asset, Total	6	1,908	(307)
Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Amounts reclassified to AOCL	3,873	3,595	(677)
Other Comprehensive Loss (Income), Prior service cost	0	0	0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	(923)	(648)	(1,052)
Total reclassifications before tax	$ 2,950	$ 2,947	$ (1,729)