Fair Value Measurements - Assets at Fair Value Recurring Basis (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | $ 1,732 | $ 1,798 |
Other securities | 68 | 86 |
U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 11 | 11 |
Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 89 | 92 |
Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,055 | 1,098 |
RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 217 | 217 |
CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 63 | 57 |
CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 82 | 85 |
Level 1 | | |
Assets | | |
Cash equivalents in securities | 4,185 | 1,159 |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 35 | 45 |
Level 2 | | |
Assets | | |
Cash equivalents in securities | 18 | 68 |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 1,763 | 1,835 |
Level 3 | | |
Assets | | |
Cash equivalents in securities | 0 | 0 |
Investments, Debt and Equity Securities [Abstract] | | |
Total investment securities | 2 | 4 |
Fair Value, Measurements, Recurring | | |
Assets | | |
Cash equivalents in mutual funds | 3,951 | 775 |
Cash equivalents in securities | 18 | 68 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,732 | 1,798 |
Other securities | 68 | 86 |
Total investment securities | 1,800 | 1,884 |
Restricted cash in mutual funds | 573 | 403 |
Total | 6,342 | 3,130 |
Fair Value, Measurements, Recurring | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 33 | 40 |
Fair Value, Measurements, Recurring | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 11 | 11 |
Fair Value, Measurements, Recurring | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 89 | 92 |
Fair Value, Measurements, Recurring | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 76 | 91 |
Fair Value, Measurements, Recurring | Non-U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 139 | 147 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,055 | 1,098 |
Other securities | 18 | 24 |
Fair Value, Measurements, Recurring | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 217 | 217 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 63 | 57 |
Fair Value, Measurements, Recurring | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 82 | 85 |
Other securities | 13 | 14 |
Fair Value, Measurements, Recurring | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 15 | 19 |
Fair Value, Measurements, Recurring | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 19 | 26 |
Fair Value, Measurements, Recurring | Other long-term investments | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 1 | | |
Assets | | |
Cash equivalents in mutual funds | 3,951 | 775 |
Cash equivalents in securities | 0 | 0 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 4 | 5 |
Other securities | 31 | 40 |
Total investment securities | 35 | 45 |
Restricted cash in mutual funds | 573 | 403 |
Total | 4,559 | 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 | 4 | 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 | 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 | 12 | 14 |
Fair Value, Measurements, Recurring | Level 1 | Common stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 19 | 26 |
Fair Value, Measurements, Recurring | Level 1 | Other long-term investments | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 2 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Cash equivalents in securities | 18 | 68 |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,727 | 1,793 |
Other securities | 36 | 42 |
Total investment securities | 1,763 | 1,835 |
Restricted cash in mutual funds | 0 | 0 |
Total | 1,781 | 1,903 |
Fair Value, Measurements, Recurring | Level 2 | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 33 | 37 |
Fair Value, Measurements, Recurring | Level 2 | U.S. government and government sponsored entities | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 11 | 11 |
Fair Value, Measurements, Recurring | Level 2 | Obligations of states, municipalities, and political subdivisions | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 89 | 92 |
Fair Value, Measurements, Recurring | Level 2 | Commercial paper | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 76 | 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 | 139 | 147 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 2 | Corporate debt | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 1,051 | 1,093 |
Other securities | 18 | 23 |
Fair Value, Measurements, Recurring | Level 2 | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 217 | 217 |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 2 | CMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 63 | 57 |
Fair Value, Measurements, Recurring | Level 2 | CDO/ABS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 81 | 85 |
Other securities | 13 | 12 |
Fair Value, Measurements, Recurring | Level 2 | Preferred stock | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 3 | 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 | 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 | 1 | 0 |
Other securities | 1 | 4 |
Total investment securities | 2 | 4 |
Restricted cash in mutual funds | 0 | 0 |
Total | 2 | 4 |
Fair Value, Measurements, Recurring | Level 3 | Total bonds | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | 0 | 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 | 0 | 0 |
Other securities | 0 | 1 |
Fair Value, Measurements, Recurring | Level 3 | RMBS | | |
Investments, Debt and Equity Securities [Abstract] | | |
Available-for-sale securities | 0 | 0 |
Other securities | 0 | 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 | 1 | 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 | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Other long-term investments | | |
Investments, Debt and Equity Securities [Abstract] | | |
Other securities | $ 1 | $ 1 |