To Be Announced RMBS (Details) - USD ($) $ in Thousands |
Jun. 30, 2018 |
Dec. 31, 2017 |
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| To Be Announced Securities [Line Items] | ||||||||||||||||||||
| Investments Fair Value | $ 2,625,471 | $ 2,071,707 | ||||||||||||||||||
| Financial Instruments Sold, Not yet Purchased, at Fair Value | (882,146) | (642,240) | ||||||||||||||||||
| Payable for securities purchased | (431,024) | (202,703) | ||||||||||||||||||
| To Be Announced Fixed Rate Agency Securities [Member] | ||||||||||||||||||||
| To Be Announced Securities [Line Items] | ||||||||||||||||||||
| Investments Fair Value | 317,013 | [1],[2],[3] | 123,680 | [4],[5],[6] | ||||||||||||||||
| Investment Owned, Balance, Principal Amount | 306,847 | 118,806 | ||||||||||||||||||
| Receivable for securities sold | 617,433 | 460,666 | ||||||||||||||||||
| TBA - Fixed Rate Agency Securities Sold Short [Member] | ||||||||||||||||||||
| To Be Announced Securities [Line Items] | ||||||||||||||||||||
| Financial Instruments Sold, Not yet Purchased, at Fair Value | (618,665) | [2],[7] | (460,189) | [5],[8] | ||||||||||||||||
| Investment Sold, Not yet Purchased, Balance, Principal Amount | (608,800) | (442,197) | ||||||||||||||||||
| Payable for securities purchased | (317,088) | (123,918) | ||||||||||||||||||
| Investment Owned At Fair Value, Net | $ (301,652) | $ (336,509) | ||||||||||||||||||
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