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Marketable Securities (Tables)
 6 Months Ended
Jun. 30, 2021
Marketable Securities [Abstract]  
Investments Available For Sale
Available For Sale and Equity Securities on the accompanying Consolidated Balance Sheets consist of: 
 At June 30, 2021
 Cost
Amount		Gross
Unrealized
Gains		Gross
Unrealized
(Losses)		Fair
Value
Available-for-sale debt securities$115,818 $1,490 $— $117,308 
SEI-sponsored mutual funds6,896 666 — 7,562 
Equities and other mutual funds4,933 236 — 5,169 
$127,647 $2,392 $— $130,039 
 At December 31, 2020
 Cost
Amount		Gross
Unrealized
Gains		Gross
Unrealized
(Losses)		Fair
Value
Available-for-sale debt securities$91,262 $2,015 $— $93,277 
SEI-sponsored mutual funds6,866 382 — 7,248 
Equities and other mutual funds4,421 473 — 4,894 
$102,549 $2,870 $— $105,419