Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,275	$ 2,775
Interest cost	3,782	4,863
Expected return on plan assets	(11,300)	(11,300)
Amortization of net loss and prior service costs	1,461	2,430
Net periodic benefit cost (reduction of cost)	(2,782)	(1,232)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	29
Interest cost	164	128
Amortization of net loss and prior service costs	(229)	(227)
Net periodic benefit cost (reduction of cost)	$ (43)	$ (70)