Employee Benefit Plans (Schedule of SERP's Pension Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	$ 3,733	$ 3,559
Interest cost on projected benefit obligations	3,621	3,505
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the year	143	133	$ 140
Interest cost on projected benefit obligations	360	367	348
Net amortization and deferral	324	295	654
Net periodic pension cost	$ 827	$ 795	$ 1,142