| Fair Value Measurements |
Fair Value Measurements We categorize assets and liabilities recorded at fair value based upon the level of judgment associated with inputs used to measure their fair value. The levels of the fair value hierarchy are as follows: | | • | Level 1—Inputs are unadjusted quoted prices in active markets for identical assets or liabilities. |
| | • | Level 2—Inputs are quoted prices for similar assets and liabilities in active markets or quoted prices for identical or similar instruments in markets that are not active and model-derived valuations in which all significant inputs and significant value drivers are observable in active markets. |
| | • | Level 3—Inputs are unobservable inputs based on our own assumptions and valuation techniques used to measure assets and liabilities at fair value. The inputs require significant management judgment or estimation. |
Our assessment of the significance of a particular input to the fair value measurement requires judgment and may affect the valuation of fair value assets and liabilities and their placement within the fair value hierarchy levels. We value our investments using quoted prices for identical instruments in active markets when available. If we are unable to obtain quoted prices for identical instruments in active markets, we value our investments using quoted market prices for comparable instruments. To date, all of our investments can be valued using one of these two methodologies. The following tables present the fair value of our financial assets using the fair value hierarchy as of March 31, 2018 and December 31, 2017: | | | | | | | | | | | | | | | | | |
|
| March 31, 2018 |
|
| Level 1 |
| Level 2 |
| Level 3 |
| Total |
|
| (in thousands) | Cash and cash equivalents |
| | | | | | | | Money market funds |
| $ | 553,667 |
|
| $ | — |
|
| $ | — |
|
| $ | 553,667 |
| Short-term investments |
| | | | | | | | U.S. treasury securities | | — |
| | 158,472 |
| | — |
| | 158,472 |
| U.S. agency securities | | — |
| | 21,313 |
| | — |
| | 21,313 |
| Corporate bonds | | — |
| | 61,867 |
| | — |
| | 61,867 |
| Long-term investments | | | | | | | | | U.S. treasury securities |
| — |
|
| 99,060 |
|
| — |
|
| 99,060 |
| U.S. agency securities |
| — |
|
| 8,505 |
|
| — |
|
| 8,505 |
| Corporate bonds | | — |
| | 49,932 |
| | — |
| | 49,932 |
| Total |
| $ | 553,667 |
|
| $ | 399,149 |
|
| $ | — |
|
| $ | 952,816 |
|
| | | | | | | | | | | | | | | | | | | | December 31, 2017 | | | Level 1 | | Level 2 | | Level 3 | | Total | | | (in thousands) | Cash and cash equivalents | | | | | | | | | Money market funds | | $ | 582,835 |
| | $ | — |
| | $ | — |
| | $ | 582,835 |
| Commercial paper | | — |
| | 8,984 |
| | — |
| | 8,984 |
| Short-term investments | | | | | | | | | Commercial paper | | — |
| | 9,970 |
| | — |
| | 9,970 |
| U.S. treasury securities | | — |
| | 160,085 |
| | — |
| | 160,085 |
| U.S. agency securities | | — |
| | 9,893 |
| | — |
| | 9,893 |
| Corporate bonds | | — |
| | 46,839 |
| | — |
| | 46,839 |
| Long-term investments | | | | | | | | | U.S. treasury securities | | — |
| | 79,169 |
| | — |
| | 79,169 |
| U.S. agency securities | | — |
| | 18,468 |
| | — |
| | 18,468 |
| Corporate bonds | | — |
| | 50,727 |
| | — |
| | 50,727 |
| Total | | $ | 582,835 |
| | $ | 384,135 |
| | $ | — |
| | $ | 966,970 |
|
We did not have any investments in prime money market funds as of March 31, 2018. We had no financial assets or liabilities measured using Level 3 inputs as of March 31, 2018 and December 31, 2017. |