Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,208	$ 2,735	$ 6,483	$ 5,510
Interest cost	4,641	4,661	8,423	9,524
Expected return on plan assets	(12,797)	(11,300)	(24,097)	(22,600)
Amortization of net loss and prior service costs	1,853	2,552	3,314	4,982
Net periodic benefit cost (reduction of cost)	(3,095)	(1,352)	(5,877)	(2,584)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	22	50	51
Interest cost	107	164	271	292
Amortization of net loss and prior service costs	(141)	(228)	(370)	(455)
Net periodic benefit cost (reduction of cost)	$ (6)	$ (42)	$ (49)	$ (112)