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 | Jun. 30, 2020 | Dec. 31, 2019 |
Fair Value, Financial Instruments [Line Items] | | |
Investments | $ 453 | $ 342 |
Total cash equivalents and investments and restricted cash equivalents and investments | 1,780 | 1,798 |
Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Cash equivalents | 286 | 92 |
Investments | 204 | 194 |
Restricted cash equivalents | 105 | 70 |
Restricted investments | 249 | 148 |
Total cash equivalents and investments and restricted cash equivalents and investments | 844 | 504 |
Fair Value Measurement on a Recurring Basis | Level 1 | | |
Fair Value, Financial Instruments [Line Items] | | |
Cash equivalents | 286 | 89 |
Investments | 0 | 0 |
Restricted cash equivalents | 105 | 56 |
Restricted investments | 0 | 0 |
Total cash equivalents and investments and restricted cash equivalents and investments | 391 | 145 |
Fair Value Measurement on a Recurring Basis | Level 2 | | |
Fair Value, Financial Instruments [Line Items] | | |
Cash equivalents | 0 | 3 |
Investments | 204 | 194 |
Restricted cash equivalents | 0 | 14 |
Restricted investments | 249 | 148 |
Total cash equivalents and investments and restricted cash equivalents and investments | 453 | 359 |
Money market mutual funds | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Cash equivalents | 286 | 89 |
Restricted cash equivalents | 91 | 42 |
Money market mutual funds | Fair Value Measurement on a Recurring Basis | Level 1 | | |
Fair Value, Financial Instruments [Line Items] | | |
Cash equivalents | 286 | 89 |
Restricted cash equivalents | 91 | 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 | 14 | 14 |
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 1 | | |
Fair Value, Financial Instruments [Line Items] | | |
Restricted cash equivalents | 14 | 14 |
Commercial paper | Fair Value Measurement on a Recurring Basis | Level 2 | | |
Fair Value, Financial Instruments [Line Items] | | |
Restricted cash equivalents | 0 | 0 |
U.S. treasuries | | |
Fair Value, Financial Instruments [Line Items] | | |
Investments | 285 | 163 |
U.S. treasuries | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Investments | 86 | 53 |
Restricted investments | 199 | 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 | 86 | 53 |
Restricted investments | 199 | 110 |
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 | 114 | 124 |
Corporate bonds | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Investments | 83 | 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 | 83 | 96 |
Restricted investments | 31 | 28 |
U.S. government agencies and government- sponsored agencies | | |
Fair Value, Financial Instruments [Line Items] | | |
Investments | 25 | 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 | 19 | 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 | 19 | 9 |
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 |