Servicing Assets - Narrative (Details) - USD ($) $ in Millions |
9 Months Ended | ||
|---|---|---|---|
Sep. 30, 2018 |
Sep. 30, 2017 |
Dec. 31, 2017 |
|
| Servicing Assets at Fair Value [Line Items] | |||
| Unpaid principal balance of loans serviced for others requiring recognition of a servicing asset | $ 2,700 | $ 2,440 | |
| Unpaid principal balances of loan serviced for others | $ 2,780 | $ 2,540 | |
| Weighted Average | |||
| Servicing Assets at Fair Value [Line Items] | |||
| Fair value of servicing rights, discount rate | 16.10% | 12.60% | |
| Fair value of servicing rights, prepayment rate | 11.10% | 7.30% | |