Fair Value Measurements - Assets at Fair Value Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017 |
Assets | | |
Total available-for-sale securities | $ 1,611 | $ 1,578 |
Investment securities: | | |
Other securities | 96 | 118 |
Debt securities | 51 | 74 |
Total investment securities | 1,707 | 1,697 |
U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 20 | 28 |
Obligations of states, municipalities, and political subdivisions | | |
Assets | | |
Total available-for-sale securities | 95 | 135 |
Certificates of deposit and commercial paper | | |
Assets | | |
Total available-for-sale securities | 52 | 60 |
Corporate debt | | |
Assets | | |
Total available-for-sale securities | 1,014 | 948 |
Investment securities: | | |
Debt securities | 46 | 68 |
RMBS | | |
Assets | | |
Total available-for-sale securities | 122 | 99 |
Investment securities: | | |
Debt securities | 1 | 1 |
CMBS | | |
Assets | | |
Total available-for-sale securities | 73 | 87 |
CDO/ABS | | |
Assets | | |
Total available-for-sale securities | 94 | 96 |
Investment securities: | | |
Debt securities | 3 | 4 |
Common stock | Portion at Other than Fair Value Measurement | | |
Assets | | |
Total available-for-sale securities | | 1 |
Level 1 | | |
Assets | | |
Cash equivalents in securities | 1,184 | 904 |
Investment securities: | | |
Total investment securities | 37 | 36 |
Level 2 | | |
Assets | | |
Cash equivalents in securities | 33 | 54 |
Investment securities: | | |
Total investment securities | 1,665 | 1,654 |
Level 3 | | |
Assets | | |
Cash equivalents in securities | 0 | 0 |
Investment securities: | | |
Total investment securities | 5 | 7 |
Fair Value, Measurements, Recurring | | |
Assets | | |
Cash equivalents in mutual funds | 487 | 688 |
Total available-for-sale securities | 1,611 | 1,578 |
Investment securities: | | |
Other securities | 96 | 118 |
Total investment securities | 1,707 | 1,696 |
Restricted cash in mutual funds | 480 | 484 |
Total | 2,707 | 2,922 |
Fair Value, Measurements, Recurring | Bonds: | | |
Investment securities: | | |
Other securities | 51 | 74 |
Fair Value, Measurements, Recurring | U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 20 | 28 |
Fair Value, Measurements, Recurring | Obligations of states, municipalities, and political subdivisions | | |
Assets | | |
Total available-for-sale securities | 95 | 135 |
Fair Value, Measurements, Recurring | Certificates of deposit and commercial paper | | |
Assets | | |
Total available-for-sale securities | 52 | 60 |
Fair Value, Measurements, Recurring | Non-U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 141 | 125 |
Investment securities: | | |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Corporate debt | | |
Assets | | |
Total available-for-sale securities | 1,014 | 948 |
Investment securities: | | |
Other securities | 46 | 68 |
Fair Value, Measurements, Recurring | RMBS | | |
Assets | | |
Total available-for-sale securities | 122 | 99 |
Investment securities: | | |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | CMBS | | |
Assets | | |
Total available-for-sale securities | 73 | 87 |
Fair Value, Measurements, Recurring | CDO/ABS | | |
Assets | | |
Total available-for-sale securities | 94 | 96 |
Investment securities: | | |
Other securities | 3 | 4 |
Fair Value, Measurements, Recurring | Preferred stock | | |
Investment securities: | | |
Debt securities | 20 | 20 |
Fair Value, Measurements, Recurring | Common stock | | |
Investment securities: | | |
Debt securities | 24 | 23 |
Fair Value, Measurements, Recurring | Other long-term investments | | |
Investment securities: | | |
Debt securities | 1 | 1 |
Fair Value, Measurements, Recurring | Cash equivalents in securities | | |
Assets | | |
Cash equivalents in securities | 33 | 54 |
Fair Value, Measurements, Recurring | Level 1 | | |
Assets | | |
Cash equivalents in mutual funds | 487 | 688 |
Total available-for-sale securities | 0 | 0 |
Investment securities: | | |
Other securities | 37 | 36 |
Total investment securities | 37 | 36 |
Restricted cash in mutual funds | 480 | 484 |
Total | 1,004 | 1,208 |
Fair Value, Measurements, Recurring | Level 1 | Bonds: | | |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Obligations of states, municipalities, and political subdivisions | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit and commercial paper | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Non-U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Corporate debt | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | RMBS | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | CMBS | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | CDO/ABS | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Preferred stock | | |
Investment securities: | | |
Debt securities | 13 | 13 |
Fair Value, Measurements, Recurring | Level 1 | Common stock | | |
Investment securities: | | |
Debt securities | 24 | 23 |
Fair Value, Measurements, Recurring | Level 1 | Other long-term investments | | |
Investment securities: | | |
Debt securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents in securities | | |
Assets | | |
Cash equivalents in securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 2 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Total available-for-sale securities | 1,608 | 1,575 |
Investment securities: | | |
Other securities | 57 | 79 |
Total investment securities | 1,665 | 1,654 |
Restricted cash in mutual funds | 0 | 0 |
Total | 1,698 | 1,708 |
Fair Value, Measurements, Recurring | Level 2 | Bonds: | | |
Investment securities: | | |
Other securities | 50 | 72 |
Fair Value, Measurements, Recurring | Level 2 | U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 20 | 28 |
Fair Value, Measurements, Recurring | Level 2 | Obligations of states, municipalities, and political subdivisions | | |
Assets | | |
Total available-for-sale securities | 95 | 135 |
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit and commercial paper | | |
Assets | | |
Total available-for-sale securities | 52 | 60 |
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 141 | 125 |
Investment securities: | | |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 2 | Corporate debt | | |
Assets | | |
Total available-for-sale securities | 1,012 | 946 |
Investment securities: | | |
Other securities | 45 | 66 |
Fair Value, Measurements, Recurring | Level 2 | RMBS | | |
Assets | | |
Total available-for-sale securities | 122 | 99 |
Investment securities: | | |
Other securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 2 | CMBS | | |
Assets | | |
Total available-for-sale securities | 73 | 87 |
Fair Value, Measurements, Recurring | Level 2 | CDO/ABS | | |
Assets | | |
Total available-for-sale securities | 93 | 95 |
Investment securities: | | |
Other securities | 3 | 4 |
Fair Value, Measurements, Recurring | Level 2 | Preferred stock | | |
Investment securities: | | |
Debt securities | 7 | 7 |
Fair Value, Measurements, Recurring | Level 2 | Common stock | | |
Investment securities: | | |
Debt securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 2 | Other long-term investments | | |
Investment securities: | | |
Debt securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents in securities | | |
Assets | | |
Cash equivalents in securities | 33 | 54 |
Fair Value, Measurements, Recurring | Level 3 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Total available-for-sale securities | 3 | 3 |
Investment securities: | | |
Other securities | 2 | 3 |
Total investment securities | 5 | 6 |
Restricted cash in mutual funds | 0 | 0 |
Total | 5 | 6 |
Fair Value, Measurements, Recurring | Level 3 | Bonds: | | |
Investment securities: | | |
Other securities | 1 | 2 |
Fair Value, Measurements, Recurring | Level 3 | U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Obligations of states, municipalities, and political subdivisions | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit and commercial paper | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Non-U.S. government and government sponsored entities | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Corporate debt | | |
Assets | | |
Total available-for-sale securities | 2 | 2 |
Investment securities: | | |
Other securities | 1 | 2 |
Fair Value, Measurements, Recurring | Level 3 | RMBS | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | CMBS | | |
Assets | | |
Total available-for-sale securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | CDO/ABS | | |
Assets | | |
Total available-for-sale securities | 1 | 1 |
Investment securities: | | |
Other securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Preferred stock | | |
Investment securities: | | |
Debt securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Common stock | | |
Investment securities: | | |
Debt securities | 0 | 0 |
Fair Value, Measurements, Recurring | Level 3 | Other long-term investments | | |
Investment securities: | | |
Debt securities | 1 | 1 |
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents in securities | | |
Assets | | |
Cash equivalents in securities | $ 0 | $ 0 |