Summary of Significant Accounting Policies (Summary Of The Amortized Costs And Fair Values Of Marketable Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
MARKETABLE SECURITIES, Amortized Cost	$ 134,572	$ 144,361
MARKETABLE SECURITIES, Fair Value	134,598	144,252
MARKETABLE SECURITIES, Gross Unrealized Gains	44
MARKETABLE SECURITIES, Gross Unrealized Losses	18	109
Tax-Exempt Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
MARKETABLE SECURITIES, Amortized Cost	40,156	30,103
MARKETABLE SECURITIES, Fair Value	40,182	29,994
MARKETABLE SECURITIES, Gross Unrealized Gains	44
MARKETABLE SECURITIES, Gross Unrealized Losses	18	109
Variable Rate Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
MARKETABLE SECURITIES, Amortized Cost	94,416	114,258
MARKETABLE SECURITIES, Fair Value	$ 94,416	$ 114,258

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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-Publisher FASB
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