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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 | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Fair Value, Financial Instruments [Line Items] |  |
Total investments | $ 445 | $ 451
Total cash equivalents and investments and restricted cash equivalents and investments | 1,966 | 2,094
Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents | 127 | 13
Total investments | 202 | 195
Total restricted cash equivalents | 96 | 99
Total restricted investments | 243 | 256
Total cash equivalents and investments and restricted cash equivalents and investments | 668 | 563
Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents | 127 | 2
Total investments | 0 | 0
Total restricted cash equivalents | 95 | 99
Total restricted investments | 0 | 0
Total cash equivalents and investments and restricted cash equivalents and investments | 222 | 101
Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents | 0 | 11
Total investments | 202 | 195
Total restricted cash equivalents | 1 | 0
Total restricted investments | 243 | 256
Total cash equivalents and investments and restricted cash equivalents and investments | 446 | 462
Money market mutual funds | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents | 127 | 2
Total restricted cash equivalents | 95 | 99
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents | 127 | 2
Total restricted cash equivalents | 95 | 99
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents | 0 | 0
Total restricted cash equivalents | 0 | 0
U.S. treasuries | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents |  | 11
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents |  | 0
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total cash equivalents |  | 11
Certificate of deposits | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents | 1 |
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents | 0 |
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents | 1 |
Commercial paper | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents | 0 |
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents | 0 |
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted cash equivalents | 0 |
Asset-backed securities |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 25 | 24
Asset-backed securities | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 25 | 24
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 0 | 0
Asset-backed securities | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 25 | 24
Corporate bonds |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 123 | 128
Corporate bonds | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 89 | 93
Total restricted investments | 34 | 35
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 0 | 0
Total restricted investments | 0 | 0
Corporate bonds | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 89 | 93
Total restricted investments | 34 | 35
U.S. government agencies and government- sponsored agencies |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 23 | 28
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 5 | 5
Total restricted investments | 18 | 23
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 0 | 0
Total 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] |  |
Total investments | 5 | 5
Total restricted investments | 18 | 23
U.S. treasuries |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 259 | 265
U.S. treasuries | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 77 | 67
Total restricted investments | 182 | 198
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 0 | 0
Total restricted investments | 0 | 0
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 77 | 67
Total restricted investments | 182 | 198
Certificate of deposits |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 9 | 0
Certificate of deposits | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted investments | 9 |
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted investments | 0 |
Certificate of deposits | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total restricted investments | 9 |
Other debt securities |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 6 | 6
Other debt securities | Fair Value Measurement on a Recurring Basis |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 6 | 6
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 1 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | 0 | 0
Other debt securities | Fair Value Measurement on a Recurring Basis | Level 2 |  |
Fair Value, Financial Instruments [Line Items] |  |
Total investments | $ 6 | $ 6

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