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Marketable Securities (Tables)
 9 Months Ended
Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]  
Schedule of available-for-sale securities The cost basis and fair value of available-for-sale securities as of September 30, 2021 (unaudited) and December 31, 2020 are presented below (in thousands):
As of September 30, 2021 (Unaudited)
Amortized
Cost		Unrealized
Gain		Unrealized
Loss		Estimated
Fair Value
Marketable securities - restricted
Corporate debt securities$3,356 $— $— $3,356 
U.S. treasury and agency securities25,274 — (6)25,268 
Commercial paper12,088 — — 12,088 
Asset backed securities9,083 — (3)9,080 
Total marketable securities - restricted$49,801 $— $(9)$49,792 
As of December 31, 2020
Amortized
Cost		Unrealized
Gain		Unrealized
Loss		Estimated
Fair Value
Marketable securities - restricted
Corporate debt securities$5,938 $17 $— $5,955 
U.S. treasury securities6,994 — — 6,994 
Commercial paper8,791 — — 8,791 
Asset backed securities2,911 — — 2,911 
Total marketable securities - restricted$24,634 $17 $— $24,651 
Schedule of amortized cost and estimated fair value of marketable securities
The amortized cost and estimated fair value of marketable securities as of September 30, 2021 (unaudited) and December 31, 2020 and are shown below by contractual maturity (in thousands):
As of September 30,
2021 (unaudited)
Amortized
Cost		Estimated
Fair Value
Due within one year$29,171 $29,193 
Due between one to five years20,630 20,599 
$49,801 $49,792 
As of December 31,
2020
Amortized
Cost		Estimated
Fair Value
Due within one year$21,603 $21,629 
Due between one to five years3,031 3,022 
$24,634 $24,651