Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 1,141	$ 1,457
Interest cost	3,452	3,208
Expected return on assets	(5,547)	(6,568)
Amortization of:
Prior service credit	(28)	(11)
Actuarial loss	1,729	1,298
Net periodic benefit (credit) cost	747	(616)
Postretirement Benefits [Member]
Components of net periodic benefit cost:
Service cost	219	332
Interest cost	577	658
Expected return on assets	(681)	(802)
Amortization of:
Transition obligation	1	1
Prior service credit	(667)	(667)
Actuarial loss	106	128
Net periodic benefit (credit) cost	$ (445)	$ (350)