Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Deferred Net Periodic Benefit Cost [Abstract]
Deferred pension and OPRB costs	$ 1,300		$ 4,600
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	1,040	$ 1,431	3,402	$ 4,292
Interest cost	3,170	3,215	10,364	9,645
Expected return on assets	(5,054)	(6,560)	(16,523)	(19,680)
Amortization of:
Prior service credit	(26)	(11)	(85)	(33)
Actuarial loss	1,637	1,319	5,351	3,958
Net periodic benefit (credit) cost	767	(606)	2,509	(1,818)
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	276	320	829	962
Interest cost	721	653	2,164	1,959
Expected return on assets	(677)	(808)	(2,034)	(2,424)
Amortization of:
Transition obligation	1	1	2	2
Prior service credit	(664)	(667)	(1,994)	(2,001)
Actuarial loss	100	122	300	366
Net periodic benefit (credit) cost	$ (243)	$ (379)	$ (733)	$ (1,136)