The following table summarizes our financial instruments by significant categories and fair value measurement on a recurring basis as of March 31, 2018 and December 31, 2017. | | | | | | | | | | | (in millions) | Level 1 | Level 2 | Total | March 31, 2018 | | | | Restricted cash equivalents: | | | | Money market mutual funds | $ | 224 |
| $ | — |
| $ | 224 |
| Commercial paper | 20 |
| — |
| 20 |
| Total restricted cash equivalents | 244 |
| — |
| 244 |
| Restricted investments: | | | | U.S. Treasuries | 23 |
| — |
| 23 |
| Exchange traded fund | 1 |
| — |
| 1 |
| Certificate of deposit | — |
| 2 |
| 2 |
| Total restricted investments | 24 |
| 2 |
| 26 |
| Total restricted cash equivalents and investments | $ | 268 |
| $ | 2 |
| $ | 270 |
| | | | | December 31, 2017 | | | | Restricted cash equivalents: | | | | Money market mutual funds | $ | 199 |
| $ | — |
| $ | 199 |
| Commercial paper | 21 |
| — |
| 21 |
| Total restricted cash equivalents | 220 |
| — |
| 220 |
| Restricted investments: | | | | U.S. Treasuries | 37 |
| — |
| 37 |
| Exchange traded fund | 1 |
| — |
| 1 |
| Certificate of deposit | — |
| 2 |
| 2 |
| Total restricted investments | 38 |
| 2 |
| 40 |
| Total restricted cash equivalents and investments | $ | 258 |
| $ | 2 |
| $ | 260 |
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