Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Defined Benefit Plan Deferred Pension And Other Postretirement Benefit Plan Costs Recovered [Abstract]
Deferred (recognized) pension and OPEB costs	$ (200)	$ 1,300	$ (700)	$ 4,600	$ 6,200
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	1,324	1,040	3,972	3,402
Interest cost	3,052	3,170	9,157	10,364
Expected return on assets	(6,807)	(5,054)	(20,422)	(16,523)
Amortization of:
Prior service credit	(31)	(26)	(93)	(85)
Actuarial loss	1,339	1,637	4,018	5,351
Net periodic benefit (credit) cost	(1,123)	767	(3,368)	2,509
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	316	276	948	829
Interest cost	570	721	1,709	2,164
Expected return on assets	(789)	(677)	(2,366)	(2,034)
Amortization of:
Transition obligation	1	1	2	2
Prior service credit	(667)	(664)	(2,001)	(1,994)
Actuarial loss	55	100	165	300
Net periodic benefit (credit) cost	$ (514)	$ (243)	$ (1,543)	$ (733)