Defined Pension Plans (Schedule of Net Periodic Pension Costs) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	$ 4,294	$ 3,733
Interest cost on projected benefit obligations	3,655	3,621
Expected return on plan assets	(5,206)	(4,681)
Net amortization and deferral	1,409	1,382
Net periodic pension cost	4,152	4,055
Unfunded Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	92	143
Interest cost on projected benefit obligations	348	360
Net amortization and deferral	581	324
Net periodic pension cost	$ 1,021	$ 827