Investment Securities At Fair Value (Maturity Dates of Marketable Debt Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Market Value	$ 77,279	$ 84,814
Under 1 Year	14,766
1 Year up to 5 Years	62,513
More than 5 Years	0
U.S. Government securities
Schedule of Available-for-sale Securities [Line Items]
Market Value	28,347
Under 1 Year	4,333
1 Year up to 5 Years	24,014
More than 5 Years	0
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Market Value	40,869
Under 1 Year	9,401
1 Year up to 5 Years	31,468
More than 5 Years	0
U.S. mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Market Value	4,081
Under 1 Year	678
1 Year up to 5 Years	3,403
More than 5 Years	0
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Market Value	417
Under 1 Year	0
1 Year up to 5 Years	417
More than 5 Years	0
Index-linked U.S. bonds
Schedule of Available-for-sale Securities [Line Items]
Market Value	2,326
Under 1 Year	0
1 Year up to 5 Years	2,326
More than 5 Years	0
Foreign fixed-income securities
Schedule of Available-for-sale Securities [Line Items]
Market Value	1,239
Under 1 Year	354
1 Year up to 5 Years	885
More than 5 Years	$ 0