Fair Value Measurements - Recurring Basis (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017 |
Investment securities: | | |
Other securities | $ 8 | $ 9 |
Total investment securities | 491 | 536 |
Total Carried At Fair Value | | |
Assets | | |
Cash equivalents in securities | 1,431 | 244 |
Not carried at fair value | Common Stock | | |
Investment securities: | | |
Available-for-sale securities | | 1 |
Level 1 | | |
Assets | | |
Cash equivalents in securities | 1,431 | 240 |
Level 2 | | |
Assets | | |
Cash equivalents in securities | 0 | 4 |
Level 3 | | |
Assets | | |
Cash equivalents in securities | 0 | 0 |
Recurring basis | | |
Investment securities: | | |
Other securities | 8 | |
Recurring basis | Preferred stock | | |
Investment securities: | | |
Trading securities, equity | 6 | 5 |
Recurring basis | Other long-term investments | | |
Investment securities: | | |
Trading securities, equity | 1 | |
Recurring basis | Common Stock | | |
Investment securities: | | |
Trading securities, equity | | 1 |
Recurring basis | Total Carried At Fair Value | | |
Assets | | |
Cash equivalents in mutual funds | 1,355 | 142 |
Investment securities: | | |
Available-for-sale securities | 483 | |
Other securities | | 9 |
Total investment securities | 491 | 535 |
Restricted cash in mutual funds | 213 | 159 |
Total | 2,059 | 840 |
Recurring basis | Total Carried At Fair Value | U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 17 | 17 |
Recurring basis | Total Carried At Fair Value | Obligations of states, municipalities, and political subdivisions | | |
Investment securities: | | |
Available-for-sale securities | 65 | 70 |
Recurring basis | Total Carried At Fair Value | Non-U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 4 | 4 |
Recurring basis | Total Carried At Fair Value | Corporate debt | | |
Investment securities: | | |
Available-for-sale securities | 293 | 324 |
Other securities | 1 | 3 |
Recurring basis | Total Carried At Fair Value | RMBS | | |
Investment securities: | | |
Available-for-sale securities | 38 | 35 |
Recurring basis | Total Carried At Fair Value | CMBS | | |
Investment securities: | | |
Available-for-sale securities | 23 | 23 |
Recurring basis | Total Carried At Fair Value | CDO/ABS | | |
Investment securities: | | |
Available-for-sale securities | 43 | 53 |
Recurring basis | Total Carried At Fair Value | Total bonds | | |
Investment securities: | | |
Available-for-sale securities | | 526 |
Other securities | 1 | 3 |
Recurring basis | Level 1 | | |
Assets | | |
Cash equivalents in mutual funds | 1,355 | 142 |
Investment securities: | | |
Available-for-sale securities | 0 | |
Other securities | 0 | 0 |
Total investment securities | 0 | 0 |
Restricted cash in mutual funds | 213 | 159 |
Total | 1,568 | 301 |
Recurring basis | Level 1 | U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 1 | Obligations of states, municipalities, and political subdivisions | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 1 | Non-U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 1 | Corporate debt | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Other securities | 0 | 0 |
Recurring basis | Level 1 | RMBS | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 1 | CMBS | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 1 | CDO/ABS | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 1 | Total bonds | | |
Investment securities: | | |
Available-for-sale securities | | 0 |
Other securities | 0 | 0 |
Recurring basis | Level 1 | Preferred stock | | |
Investment securities: | | |
Trading securities, equity | 0 | 0 |
Recurring basis | Level 1 | Other long-term investments | | |
Investment securities: | | |
Trading securities, equity | 0 | |
Recurring basis | Level 1 | Common Stock | | |
Investment securities: | | |
Trading securities, equity | | 0 |
Recurring basis | Level 2 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Investment securities: | | |
Available-for-sale securities | 483 | |
Other securities | 7 | 8 |
Total investment securities | 490 | 534 |
Restricted cash in mutual funds | 0 | 0 |
Total | 490 | 538 |
Recurring basis | Level 2 | U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 17 | 17 |
Recurring basis | Level 2 | Obligations of states, municipalities, and political subdivisions | | |
Investment securities: | | |
Available-for-sale securities | 65 | 70 |
Recurring basis | Level 2 | Non-U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 4 | 4 |
Recurring basis | Level 2 | Corporate debt | | |
Investment securities: | | |
Available-for-sale securities | 293 | 324 |
Other securities | 1 | 3 |
Recurring basis | Level 2 | RMBS | | |
Investment securities: | | |
Available-for-sale securities | 38 | 35 |
Recurring basis | Level 2 | CMBS | | |
Investment securities: | | |
Available-for-sale securities | 23 | 23 |
Recurring basis | Level 2 | CDO/ABS | | |
Investment securities: | | |
Available-for-sale securities | 43 | 53 |
Recurring basis | Level 2 | Total bonds | | |
Investment securities: | | |
Available-for-sale securities | | 526 |
Other securities | 1 | 3 |
Recurring basis | Level 2 | Preferred stock | | |
Investment securities: | | |
Trading securities, equity | 6 | 5 |
Recurring basis | Level 2 | Other long-term investments | | |
Investment securities: | | |
Trading securities, equity | 0 | |
Recurring basis | Level 2 | Common Stock | | |
Investment securities: | | |
Trading securities, equity | | 0 |
Recurring basis | Level 3 | | |
Assets | | |
Cash equivalents in mutual funds | 0 | 0 |
Investment securities: | | |
Available-for-sale securities | 0 | |
Other securities | 1 | 1 |
Total investment securities | 1 | 1 |
Restricted cash in mutual funds | 0 | 0 |
Total | 1 | 1 |
Recurring basis | Level 3 | U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 3 | Obligations of states, municipalities, and political subdivisions | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 3 | Non-U.S. government and government sponsored entities | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 3 | Corporate debt | | |
Investment securities: | | |
Available-for-sale securities | 0 | |
Other securities | 0 | 0 |
Recurring basis | Level 3 | RMBS | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 3 | CMBS | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 3 | CDO/ABS | | |
Investment securities: | | |
Available-for-sale securities | 0 | 0 |
Recurring basis | Level 3 | Total bonds | | |
Investment securities: | | |
Available-for-sale securities | | 0 |
Other securities | 0 | 0 |
Recurring basis | Level 3 | Preferred stock | | |
Investment securities: | | |
Trading securities, equity | 0 | 0 |
Recurring basis | Level 3 | Other long-term investments | | |
Investment securities: | | |
Trading securities, equity | $ 1 | |
Recurring basis | Level 3 | Common Stock | | |
Investment securities: | | |
Trading securities, equity | | 1 |
Cash equivalents in securities | Recurring basis | Total Carried At Fair Value | | |
Assets | | |
Cash equivalents in securities | | 4 |
Cash equivalents in securities | Recurring basis | Level 1 | | |
Assets | | |
Cash equivalents in securities | | 0 |
Cash equivalents in securities | Recurring basis | Level 2 | | |
Assets | | |
Cash equivalents in securities | | 4 |
Cash equivalents in securities | Recurring basis | Level 3 | | |
Assets | | |
Cash equivalents in securities | | $ 0 |