Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,735	$ 3,163	$ 8,245	$ 9,251
Interest cost	4,659	4,279	14,183	12,481
Expected return on plan assets	(11,299)	(10,375)	(33,899)	(30,244)
Amortization of net loss and prior service costs	2,553	1,366	7,535	4,057
Net periodic benefit cost (reduction of cost)	(1,352)	(1,567)	(3,936)	(4,455)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	22	28	73	91
Interest cost	165	161	457	459
Amortization of net loss and prior service costs	(229)	(95)	(684)	(338)
Net periodic benefit cost (reduction of cost)	$ (42)	$ 94	$ (154)	$ 212