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, 2021 | Dec. 31, 2020 |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | $ 561 | $ 451 |
Total cash equivalents and investments and restricted cash equivalents and investments | 2,063 | 2,094 |
Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total cash equivalents | 5 | 13 |
Total investments | 322 | 195 |
Total restricted cash equivalents | 96 | 99 |
Total restricted investments | 239 | 256 |
Total cash equivalents and investments and restricted cash equivalents and investments | 662 | 563 |
Fair Value Measurement on a Recurring Basis | Level 1 | | |
Fair Value, Financial Instruments [Line Items] | | |
Total cash equivalents | 2 | 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 | 97 | 101 |
Fair Value Measurement on a Recurring Basis | Level 2 | | |
Fair Value, Financial Instruments [Line Items] | | |
Total cash equivalents | 3 | 11 |
Total investments | 322 | 195 |
Total restricted cash equivalents | 1 | 0 |
Total restricted investments | 239 | 256 |
Total cash equivalents and investments and restricted cash equivalents and investments | 565 | 462 |
Money market mutual funds | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total cash equivalents | 2 | 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 | 2 | 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 | 3 | 11 |
Total restricted cash equivalents | 1 | |
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 1 | | |
Fair Value, Financial Instruments [Line Items] | | |
Total cash equivalents | 0 | 0 |
Total restricted cash equivalents | 0 | |
U.S. treasuries | Fair Value Measurement on a Recurring Basis | Level 2 | | |
Fair Value, Financial Instruments [Line Items] | | |
Total cash equivalents | 3 | 11 |
Total restricted cash equivalents | 1 | |
Asset-backed securities | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 48 | 24 |
Asset-backed securities | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 48 | 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 | 48 | 24 |
Corporate bonds | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 172 | 128 |
Corporate bonds | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 140 | 93 |
Total restricted investments | 32 | 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 | 140 | 93 |
Total restricted investments | 32 | 35 |
U.S. government agencies and government- sponsored agencies | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 18 | 28 |
U.S. government agencies and government- sponsored agencies | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 2 | 5 |
Total restricted investments | 16 | 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 | 2 | 5 |
Total restricted investments | 16 | 23 |
U.S. treasuries | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 301 | 265 |
U.S. treasuries | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 122 | 67 |
Total restricted investments | 179 | 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 | 122 | 67 |
Total restricted investments | 179 | 198 |
Other debt securities | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 10 | 6 |
Other debt securities | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 10 | 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 | 10 | 6 |
Certificate of deposits | | |
Fair Value, Financial Instruments [Line Items] | | |
Total investments | 12 | $ 0 |
Certificate of deposits | Fair Value Measurement on a Recurring Basis | | |
Fair Value, Financial Instruments [Line Items] | | |
Total restricted investments | 12 | |
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 | $ 12 | |