Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Deferred Pension And Other Postretirement Benefit Plan Costs Recovered [Abstract]
Deferred (recognized) pension and OPEB costs	$ (300)	$ (200)	$ (3,900)	$ (700)
Deferred savings from 2021 employee benefit plan costs	2,800		5,400
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	1,432	1,324	4,296	3,972
Interest cost	2,280	3,052	6,840	9,157
Expected return on assets	(7,372)	(6,807)	(22,115)	(20,422)
Amortization of:
Prior service credit	(31)	(31)	(93)	(93)
Actuarial loss	1,662	1,339	4,985	4,018
Net periodic benefit (credit) cost	(2,029)	(1,123)	(6,087)	(3,368)
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	362	316	1,086	948
Interest cost	387	570	1,161	1,709
Expected return on assets	(819)	(789)	(2,457)	(2,366)
Amortization of:
Transition obligation	1	1	2	2
Prior service credit	(379)	(667)	(1,138)	(2,001)
Actuarial loss	123	55	370	165
Net periodic benefit (credit) cost	$ (325)	$ (514)	$ (976)	$ (1,543)