Cash, Cash Equivalents And Marketable Securities (Summary Of The Amortized Costs And Fair Values Of Marketable Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
MARKETABLE SECURITIES, Amortized Cost	$ 102,581	$ 144,361
MARKETABLE SECURITIES, Fair Value	102,516	144,252
MARKETABLE SECURITIES, Gross Unrealized Losses	65	109
Tax-Exempt Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
MARKETABLE SECURITIES, Amortized Cost	29,606	30,103
MARKETABLE SECURITIES, Fair Value	29,541	29,994
MARKETABLE SECURITIES, Gross Unrealized Losses	65	109
Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
MARKETABLE SECURITIES, Amortized Cost	72,975	114,258
MARKETABLE SECURITIES, Fair Value	$ 72,975	$ 114,258