Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,925	$ 3,750
Interest cost	3,923	4,123
Expected return on plan assets	(9,700)	(8,750)
Amortization of net loss/ prior service cost	1,326	1,435
Net periodic benefit cost	(1,526)	558
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	35	48
Interest cost	137	180
Amortization of net loss/ prior service cost	(149)	(48)
Net periodic benefit cost	$ 23	$ 180