Fair Value Measurements - Assets at Fair Value Recurring Basis (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | $ 1,847 | $ 1,798 |
Other securities | 75 | 86 |
U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 12 | 11 |
Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 92 | 92 |
Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,218 | 1,098 |
RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 215 | 217 |
CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 58 | 57 |
CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 78 | 85 |
Level 1 | | |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 44 | 45 |
Level 2 | | |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 1,870 | 1,835 |
Level 3 | | |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 8 | 4 |
Fair Value, Measurements, Recurring | | |
Assets | | |
Cash equivalents in mutual funds | 2,018 | 775 |
Cash equivalents in securities | 17 | 68 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,847 | 1,798 |
Other securities | 75 | 86 |
Total investment securities | 1,922 | 1,884 |
Restricted cash equivalents in mutual funds | 441 | 403 |
Total | 4,398 | 3,130 |
Fair Value, Measurements, Recurring | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 35 | 40 |
Fair Value, Measurements, Recurring | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 12 | 11 |
Fair Value, Measurements, Recurring | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 92 | 92 |
Fair Value, Measurements, Recurring | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 28 | 91 |
Fair Value, Measurements, Recurring | Non-U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 146 | 147 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,218 | 1,098 |
Other securities | 17 | 24 |
Fair Value, Measurements, Recurring | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 215 | 217 |
Other securities | | 1 |
Fair Value, Measurements, Recurring | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 58 | 57 |
Fair Value, Measurements, Recurring | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 78 | 85 |
Other securities | 17 | 14 |
Fair Value, Measurements, Recurring | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 13 | 19 |
Fair Value, Measurements, Recurring | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 27 | 26 |
Fair Value, Measurements, Recurring | Other long-term investments | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | | 1 |
Fair Value, Measurements, Recurring | Level 1 | | |
Assets | | |
Cash equivalents in mutual funds | 2,018 | 775 |
Cash equivalents in securities | 0 | 0 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 5 | 5 |
Other securities | 39 | 40 |
Total investment securities | 44 | 45 |
Restricted cash equivalents in mutual funds | 441 | 403 |
Total | 2,503 | 1,223 |
Fair Value, Measurements, Recurring | Level 1 | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Non-U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 5 | 5 |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Other securities | | 0 |
Fair Value, Measurements, Recurring | Level 1 | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 13 | 14 |
Fair Value, Measurements, Recurring | Level 1 | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 26 | 26 |
Fair Value, Measurements, Recurring | Level 1 | Other long-term investments | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | | 0 |
Fair Value, Measurements, Recurring | Level 2 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Cash equivalents in securities | 17 | 68 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,836 | 1,793 |
Other securities | 34 | 42 |
Total investment securities | 1,870 | 1,835 |
Restricted cash equivalents in mutual funds | 0 | 0 |
Total | 1,887 | 1,903 |
Fair Value, Measurements, Recurring | Level 2 | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 34 | 37 |
Fair Value, Measurements, Recurring | Level 2 | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 12 | 11 |
Fair Value, Measurements, Recurring | Level 2 | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 92 | 92 |
Fair Value, Measurements, Recurring | Level 2 | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 28 | 91 |
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 146 | 147 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 2 | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,207 | 1,093 |
Other securities | 16 | 23 |
Fair Value, Measurements, Recurring | Level 2 | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 215 | 217 |
Other securities | | 1 |
Fair Value, Measurements, Recurring | Level 2 | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 58 | 57 |
Fair Value, Measurements, Recurring | Level 2 | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 78 | 85 |
Other securities | 17 | 12 |
Fair Value, Measurements, Recurring | Level 2 | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 5 |
Fair Value, Measurements, Recurring | Level 2 | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 2 | Other long-term investments | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | | 0 |
Fair Value, Measurements, Recurring | Level 3 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Cash equivalents in securities | 0 | 0 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 6 | 0 |
Other securities | 2 | 4 |
Total investment securities | 8 | 4 |
Restricted cash equivalents in mutual funds | 0 | 0 |
Total | 8 | 4 |
Fair Value, Measurements, Recurring | Level 3 | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 1 | 3 |
Fair Value, Measurements, Recurring | Level 3 | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Non-U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 6 | 0 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 3 | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Other securities | | 0 |
Fair Value, Measurements, Recurring | Level 3 | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Other securities | 0 | 2 |
Fair Value, Measurements, Recurring | Level 3 | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | $ 1 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Other long-term investments | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | | $ 1 |