Defined Pension Plans (Schedule of Net Periodic Pension Costs) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	$ 3,805	$ 3,559
Interest cost on projected benefit obligations	3,587	3,505
Expected return on plan assets	(4,680)	(4,734)
Net amortization and deferral	1,328	1,259
Net periodic pension cost	4,040	3,589
Unfunded Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	143	133
Interest cost on projected benefit obligations	361	367
Net amortization and deferral	323	295
Net periodic pension cost	$ 827	$ 795