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Marketable Securities
 9 Months Ended
Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities Marketable Securities
All marketable securities were considered available-for-sale at September 30, 2021 and December 31, 2020. On a recurring basis, the Company records its marketable securities at fair value using Level 1 or Level 2 inputs as discussed in Note 2, "Fair Value Measurements". The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type at each balance sheet date are summarized in the tables below (in thousands):
September 30, 2021
Amortized CostUnrealized Holding GainsUnrealized Holding LossesAggregate Fair Value
Short-term marketable securities:
U.S. government treasuries(1)
$472,214 $13 $(13)$472,214 
U.S. government agency securities
15,003 — — 15,003 
Corporate debt securities(2)
61,187 5 (10)61,182 
Commercial paper
96,223 — — 96,223 
Total short-term marketable securities
644,627 18 (23)644,622 
Long-term marketable securities:
U.S. government treasuries(3)
299,225 17 (173)299,069 
Corporate debt securities(4)
20,413 2 (12)20,403 
Total long-term marketable securities
319,638 19 (185)319,472 
Total
$964,265 $37 $(208)$964,094 
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(1)Unrealized holding losses on 7 securities with an aggregate fair value of $271.8 million.
(2)Unrealized holding losses on 8 securities with an aggregate fair value of $27.8 million.
(3)Unrealized holding losses on 11 securities with an aggregate fair value of $251.5 million.
(4)Unrealized holding losses on 5 securities with an aggregate fair value of $16.8 million.

December 31, 2020
Amortized CostUnrealized Holding GainsUnrealized Holding LossesAggregate Fair Value
Short-term marketable securities:
U.S. government treasuries(1)
$878,906 $44 $(12)$878,938 
U.S. government agency securities(2)
25,214 5 (2)25,217 
Corporate debt securities
27,101 79 — 27,180 
Commercial paper
31,218 — — 31,218 
Total short-term marketable securities
962,439 128 (14)962,553 
Long-term marketable securities:
U.S. government treasuries
2,561 — — 2,561 
Corporate debt securities(3)
30,147 2 (11)30,138 
Total long-term marketable securities
32,708 2 (11)32,699 
Total
$995,147 $130 $(25)$995,252 
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(1)Unrealized holding losses on 19 securities with an aggregate fair value of $369.9 million.
(2)Unrealized holding losses on 2 securities with an aggregate fair value of $10.1 million.
(3)Unrealized holding losses on 1 security with an aggregate fair value of $20.1 million.
As of September 30, 2021 and December 31, 2020, some of the Company's marketable securities were in an unrealized loss position. The Company has not recognized an allowance for credit losses as of September 30, 2021 or December 31, 2020. The Company determined that it had the ability and intent to hold all marketable securities that have been in a continuous loss position until maturity or recovery. Further, these marketable securities were initially, and continue to be, held with investment grade, high credit quality institutions. All marketable securities with unrealized losses as of each balance sheet date have been in a loss position for less than twelve months or the loss is not material.

All of the Company’s marketable securities have an effective maturity of less than two years.