Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,450	$ 3,275
Interest cost	3,460	3,782
Expected return on plan assets	(12,058)	(11,300)
Amortization of net loss and prior service costs	2,100	1,461
Net reduction of periodic benefit cost	(3,048)	(2,782)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	27	22
Interest cost	99	164
Amortization of net loss and prior service costs	(146)	(229)
Net reduction of periodic benefit cost	$ (20)	$ (43)