Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,775	$ 2,925
Interest cost	4,863	3,923
Expected return on plan assets	(11,300)	(9,700)
Amortization of net loss and prior service costs	2,430	1,326
Net periodic benefit cost (reduction of cost)	(1,232)	(1,526)
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	29	35
Interest cost	128	137
Amortization of net loss and prior service costs	(227)	(149)
Net periodic benefit cost (reduction of cost)	$ (70)	$ 23