Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Deferred Pension And Other Postretirement Benefit Plan Costs Recovered [Abstract]
Deferred (recognized) pension and OPEB costs	$ (3,400)	$ (400)
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	1,422	1,316
Interest cost	2,272	3,023
Expected return on assets	(7,375)	(6,811)
Amortization of:
Prior service credit	(31)	(31)
Actuarial loss	1,580	1,286
Net periodic benefit (credit) cost	(2,132)	(1,217)
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	351	311
Interest cost	384	573
Expected return on assets	(817)	(790)
Amortization of:
Transition obligation	1	1
Prior service credit	(380)	(667)
Actuarial loss	109	70
Net periodic benefit (credit) cost	$ (352)	$ (502)