Fair Value Measurements - Assets at Fair Value Recurring Basis (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | $ 1,867 | $ 1,847 |
Other securities | 84 | 75 |
U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 11 | 12 |
Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 88 | 92 |
Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,234 | 1,218 |
RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 195 | 215 |
CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 56 | 58 |
CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 87 | 78 |
Level 1 | | |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 52 | 44 |
Level 2 | | |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 1,891 | 1,870 |
Level 3 | | |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 8 | 8 |
Fair Value, Measurements, Recurring | | |
Assets | | |
Cash equivalents in mutual funds | 881 | 2,018 |
Cash equivalents in securities | 3 | 17 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,867 | 1,847 |
Other securities | 84 | 75 |
Total investment securities | 1,951 | 1,922 |
Restricted cash equivalents in mutual funds | 560 | 441 |
Total | 3,395 | 4,398 |
Fair Value, Measurements, Recurring | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 37 | 35 |
Fair Value, Measurements, Recurring | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 11 | 12 |
Fair Value, Measurements, Recurring | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 88 | 92 |
Fair Value, Measurements, Recurring | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 49 | 28 |
Fair Value, Measurements, Recurring | Non-U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 147 | 146 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,234 | 1,218 |
Other securities | 13 | 17 |
Fair Value, Measurements, Recurring | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 195 | 215 |
Other securities | 1 | |
Fair Value, Measurements, Recurring | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 56 | 58 |
Fair Value, Measurements, Recurring | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 87 | 78 |
Other securities | 22 | 17 |
Fair Value, Measurements, Recurring | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 18 | 13 |
Fair Value, Measurements, Recurring | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 29 | 27 |
Fair Value, Measurements, Recurring | Level 1 | | |
Assets | | |
Cash equivalents in mutual funds | 881 | 2,018 |
Cash equivalents in securities | 0 | 0 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 5 | 5 |
Other securities | 47 | 39 |
Total investment securities | 52 | 44 |
Restricted cash equivalents in mutual funds | 560 | 441 |
Total | 1,493 | 2,503 |
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 | 18 | 13 |
Fair Value, Measurements, Recurring | Level 1 | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 29 | 26 |
Fair Value, Measurements, Recurring | Level 2 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Cash equivalents in securities | 3 | 17 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,856 | 1,836 |
Other securities | 35 | 34 |
Total investment securities | 1,891 | 1,870 |
Restricted cash equivalents in mutual funds | 0 | 0 |
Total | 1,894 | 1,887 |
Fair Value, Measurements, Recurring | Level 2 | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 35 | 34 |
Fair Value, Measurements, Recurring | Level 2 | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 11 | 12 |
Fair Value, Measurements, Recurring | Level 2 | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 88 | 92 |
Fair Value, Measurements, Recurring | Level 2 | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 49 | 28 |
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 147 | 146 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 2 | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,223 | 1,207 |
Other securities | 12 | 16 |
Fair Value, Measurements, Recurring | Level 2 | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 195 | 215 |
Other securities | 1 | |
Fair Value, Measurements, Recurring | Level 2 | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 56 | 58 |
Fair Value, Measurements, Recurring | Level 2 | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 87 | 78 |
Other securities | 21 | 17 |
Fair Value, Measurements, Recurring | Level 2 | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 2 | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 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 | 6 |
Other securities | 2 | 2 |
Total investment securities | 8 | 8 |
Restricted cash equivalents in mutual funds | 0 | 0 |
Total | 8 | 8 |
Fair Value, Measurements, Recurring | Level 3 | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 2 | 1 |
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 | 6 |
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 | 1 | 0 |
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 | $ 0 | $ 1 |