December 31, 2021

Amortized

Gross

cost, as

unrealized

Estimated fair

(in thousands)

adjusted

holding gains

holding losses

value

Short-term investments:

Obligations of domestic governmental agencies (maturing between January 2022 and April 2022) (held-to-maturity)

$

15,876

$

—

$

4

$

15,872

Long-term investments:

Obligations of domestic governmental agencies (maturing between February 2023 and June 2023) (held-to-maturity)

35,533

—

160

35,373

Total short-term and long-term investment securities

$

51,409

$

—

$

164

$

51,245

December 31, 2020

Amortized cost, as adjusted

Gross unrealized holding gains

Gross unrealized holding losses

Estimated fair value

Short-term investments:

Obligations of domestic governmental agencies (maturing between January 2021 and December 2021) (held-to-maturity)

$

51,987

$

1

$

4

$

51,984

Total short-term investment securities

$

51,987

$

1

$

4

$

51,984

Long-term investments:

Obligations of domestic governmental agencies (held-to-maturity)

$

—

$

—

$

—

$

—

Total long-term investment securities

$

—

$

—

$

—

$

—