Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,735	$ 3,163	$ 5,510	$ 6,088
Interest cost	4,661	4,279	9,524	8,202
Expected return on plan assets	(11,300)	(10,169)	(22,600)	(19,869)
Amortization of net loss and prior service costs	2,552	1,365	4,982	2,691
Net periodic benefit cost (reduction of cost)	(1,352)	(1,362)	(2,584)	(2,888)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	28	51	63
Interest cost	164	161	292	298
Amortization of net loss and prior service costs	(228)	(94)	(455)	(243)
Net periodic benefit cost (reduction of cost)	$ (42)	$ 95	$ (112)	$ 118