Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,926	$ 3,208	$ 6,376	$ 6,483
Interest cost	2,970	4,641	6,430	8,423
Expected return on plan assets	(11,333)	(12,797)	(23,391)	(24,097)
Amortization of net loss and prior service costs	1,654	1,853	3,754	3,314
Special termination benefits	16,052		16,052
Net periodic benefit cost	12,269	(3,095)	9,221	(5,877)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	21	28	48	50
Interest cost	67	107	166	271
Amortization of net loss and prior service costs	(163)	(141)	(309)	(370)
Special termination benefits	4,137		4,137
Net periodic benefit cost	$ 4,062	$ (6)	$ 4,042	$ (49)