| Summary of Financial Assets Measured at Fair Value on Recurring Basis |
The following table sets forth our financial assets as of December 31, 2018 and 2017 that are measured at fair value on a recurring basis during the period:
|
|
December 31, 2018 |
|
|
|
Cost or
Amortized Cost |
|
|
Gross
Unrealized
Gains |
|
|
Gross
Unrealized
Losses |
|
|
Total Estimated
Fair Value |
|
|
|
(in thousands) |
|
|
Cash |
$ |
279,950 |
|
|
$ |
— |
|
|
$ |
— |
|
|
$ |
279,950 |
|
|
Level 1 securities: |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
U.S. government securities |
|
735,988 |
|
|
|
12 |
|
|
|
(175 |
) |
|
|
735,825 |
|
|
U.S. government agency securities |
|
181,032 |
|
|
|
4 |
|
|
|
(36 |
) |
|
|
181,000 |
|
|
Level 2 securities: |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Corporate debt securities |
|
35,819 |
|
|
|
1 |
|
|
|
(18 |
) |
|
|
35,802 |
|
|
Commercial paper |
|
33,193 |
|
|
|
— |
|
|
|
— |
|
|
|
33,193 |
|
|
Certificates of deposit |
|
13,293 |
|
|
|
— |
|
|
|
— |
|
|
|
13,293 |
|
|
Total |
$ |
1,279,275 |
|
|
$ |
17 |
|
|
$ |
(229 |
) |
|
$ |
1,279,063 |
|
|
|
December 31, 2017 |
|
|
|
Cost or
Amortized Cost |
|
|
Gross
Unrealized
Gains |
|
|
Gross
Unrealized
Losses |
|
|
Total Estimated
Fair Value |
|
|
|
(in thousands) |
|
|
Cash |
$ |
267,715 |
|
|
$ |
— |
|
|
$ |
— |
|
|
$ |
267,715 |
|
|
Level 1 securities: |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
U.S. government securities |
|
1,377,339 |
|
|
|
2 |
|
|
|
(899 |
) |
|
|
1,376,442 |
|
|
U.S. government agency securities |
|
399,098 |
|
|
|
6 |
|
|
|
(222 |
) |
|
|
398,882 |
|
|
Level 2 securities: |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Corporate debt securities |
|
— |
|
|
|
— |
|
|
|
— |
|
|
|
— |
|
|
Commercial paper |
|
— |
|
|
|
— |
|
|
|
— |
|
|
|
— |
|
|
Certificates of deposit |
|
— |
|
|
|
— |
|
|
|
— |
|
|
|
— |
|
|
Total |
$ |
2,044,152 |
|
|
$ |
8 |
|
|
$ |
(1,121 |
) |
|
$ |
2,043,039 |
|
|