Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,207	$ 2,735	$ 9,690	$ 8,245
Interest cost	3,892	4,659	12,315	14,183
Expected return on plan assets	(12,047)	(11,299)	(36,144)	(33,899)
Amortization of net loss and prior service costs	1,854	2,553	5,168	7,535
Net reduction of periodic benefit cost	(3,094)	(1,352)	(8,971)	(3,936)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	22	78	73
Interest cost	106	165	377	457
Amortization of net loss and prior service costs	(142)	(229)	(512)	(684)
Net reduction of periodic benefit cost	$ (8)	$ (42)	$ (57)	$ (154)