Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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)	$ 3,000	$ (600)	$ 3,000	$ 6,200
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	1,332	1,221	2,648	2,362
Interest cost	3,082	3,742	6,105	7,194
Expected return on assets	(6,804)	(5,922)	(13,615)	(11,469)
Amortization of:
Prior service credit	(31)	(31)	(62)	(59)
Actuarial loss	1,392	1,985	2,678	3,714
Net periodic benefit (credit) cost	(1,029)	995	(2,246)	1,742
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	321	334	632	553
Interest cost	566	865	1,139	1,442
Expected return on assets	(787)	(676)	(1,577)	(1,357)
Amortization of:
Transition obligation	1	1	1	2
Prior service credit	(667)	(662)	(1,334)	(1,330)
Actuarial loss	40	94	110	200
Net periodic benefit (credit) cost	$ (526)	$ (44)	$ (1,029)	$ (490)