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Marketable Securities (Tables)
 3 Months Ended
Mar. 31, 2021
Marketable Securities [Abstract]  
Investments Available For Sale
Available For Sale and Equity Securities on the accompanying Consolidated Balance Sheets consist of: 
 At March 31, 2021
 Cost
Amount		Gross
Unrealized
Gains		Gross
Unrealized
(Losses)		Fair
Value
Available-for-sale debt securities$100,980 $1,638 $— $102,618 
SEI-sponsored mutual funds6,872 446 — 7,318 
Equities and other mutual funds4,494 543 — 5,037 
$112,346 $2,627 $— $114,973 
 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