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Financial Instruments and Fair Value Measurements (Tables) | 3 Months Ended
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Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract] |
Summary of Financial Instruments by Significant Categories and Fair Value Measurement on a Recurring Basis | 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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- Definition Tabular disclosure of assets, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, by class that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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