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Financial Instruments and Fair Value Measurements - Summary of Financial Instruments by Significant Categories and Fair Value Measurement on a Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value, Financial Instruments [Line Items] |  |
Investments | $ 460 | $ 342
Total cash equivalents and investments and restricted cash equivalents and investments | 1,802 | 1,798
Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 7 | 92
Investments | 202 | 194
Restricted cash equivalents | 97 | 70
Restricted investments | 258 | 148
Total cash equivalents and investments and restricted cash equivalents and investments | 564 | 504
Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 7 | 89
Investments | 0 | 0
Restricted cash equivalents | 97 | 56
Restricted investments | 0 | 0
Total cash equivalents and investments and restricted cash equivalents and investments | 104 | 145
Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 0 | 3
Investments | 202 | 194
Restricted cash equivalents | 0 | 14
Restricted investments | 258 | 148
Total cash equivalents and investments and restricted cash equivalents and investments | 460 | 359
Money market mutual funds | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 7 | 89
Restricted cash equivalents | 84 | 42
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 7 | 89
Restricted cash equivalents | 84 | 42
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents | 0 | 0
Restricted cash equivalents | 0 | 0
U.S. treasuries | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents |  | 3
Restricted cash equivalents |  | 12
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents |  | 0
Restricted cash equivalents |  | 0
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Cash equivalents |  | 3
Restricted cash equivalents |  | 12
Certificate of deposit | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents |  | 2
Certificate of deposit | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents |  | 0
Certificate of deposit | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents |  | 2
Commercial paper | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 13 | 14
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 13 | 14
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted cash equivalents | 0 | 0
Asset-backed securities |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 22 | 30
Asset-backed securities | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 22 | 30
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 | 0
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 22 | 30
Corporate bonds |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 121 | 124
Corporate bonds | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 90 | 96
Restricted investments | 31 | 28
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 | 0
Restricted investments | 0 | 0
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 90 | 96
Restricted investments | 31 | 28
U.S. government agencies and government- sponsored agencies |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 20 | 14
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 6 | 5
Restricted investments | 14 | 9
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 | 0
Restricted investments | 0 | 0
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 6 | 5
Restricted investments | 14 | 9
U.S. treasuries |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 290 | 163
U.S. treasuries | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 77 | 53
Restricted investments | 213 | 110
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 | 0
Restricted investments | 0 | 0
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 77 | 53
Restricted investments | 213 | 110
Other debt securities |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 7 | 10
Other debt securities | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 7 | 10
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 0 | 0
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | 7 | 10
Certificate of deposit |  |
Fair Value, Financial Instruments [Line Items] |  |
Investments | $ 0 | 1
Certificate of deposit | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments |  | 1
Certificate of deposit | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments |  | 0
Certificate of deposit | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Restricted investments |  | $ 1

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