Defined Pension Plans (Schedule of Net Periodic Pension Costs) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	$ 4,312	$ 3,555
Interest cost on projected benefit obligations	3,903	4,103
Expected return on plan assets	(6,051)	(4,753)
Net amortization	1,890	1,559
Net periodic pension cost	4,054	4,464
Unfunded Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	121	97
Interest cost on projected benefit obligations	347	408
Net amortization	112	276
Net periodic pension cost	$ 580	$ 781