Fair Value Measurements (Fair Value Option, Fair Value and Unpaid Principal Balance) (Details) - Mortgage loans held for sale [Member] - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
|
| Fair Value, Option, Quantitative Disclosures [Line Items] | ||
| Mortgages Held-for-sale, Fair Value Disclosure | $ 1,439 | $ 439 |
| Fair Value Option Mortgages Held For Sale Aggregate Unpaid Principal | 1,362 | 425 |
| Aggregate Fair Value Less Aggregate Unpaid Principal | 77 | 14 |
| Net gains (losses) resulting from changes in fair value | $ 63 | $ 4 |