Investments In Securities - Summary of Weighted Average Lives of Available for Sale Securities (Details) - USD ($) $ in Thousands |
Sep. 30, 2020 |
Dec. 31, 2019 |
|---|---|---|
| Fair Value | ||
| Less than 1 year | $ 45 | $ 0 |
| Greater than or equal to 1 year and less than 3 years | 217,345 | 22,237 |
| Greater than or equal to 3 years and less than 5 years | 738,231 | 6,542,389 |
| Greater than or equal to 5 years | 1,196,976 | 5,377,140 |
| Total Agency Securities | 2,152,597 | 11,941,766 |
| Amortized Cost | ||
| Less than 1 year | 45 | 0 |
| Greater than or equal to 1 year and less than 3 years | 208,549 | 22,254 |
| Greater than or equal to 3 years and less than 5 years | 705,376 | 6,365,623 |
| Greater than or equal to 5 years | 1,038,430 | 5,198,416 |
| Amortized Cost | $ 1,952,400 | $ 11,586,293 |