Consolidated Statements of Cash Flows (Unaudited) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2020
Mar. 31, 2020
Sep. 30, 2019
Mar. 31, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Cash Flows From Operating Activities:              
Net Income (Loss) $ 58,386,000 $ (406,659,000) $ (60,955,000) $ (114,381,000) $ (296,525,000) $ (358,586,000)  
Adjustments to reconcile net loss to net cash and cash collateral posted to counterparties used in operating activities:              
Net amortization of premium on Agency Securities         36,922,000 35,260,000  
Realized gain on sale of Agency Securities, available for sale         (138,802,000) (1,615,000)  
Credit loss expense 0 1,012,000 0   1,012,000 0  
Agency Securities, trading (12,149,000)   0   (20,060,000) 0  
Loss on short sale of U.S. Treasury Securities         414,000 0  
Stock based compensation         3,051,000 2,039,000  
Changes in operating assets and liabilities:              
Increase (decrease) in accrued interest receivable         21,836,000 (14,220,000)  
Increase (decrease) in prepaid and other assets         6,715,000 (111,000)  
Change in derivatives, at fair value         (44,392,000) 195,421,000  
Increase (decrease) in accrued interest payable- repurchase agreements         (30,643,000) 3,736,000  
Increase in accounts payable and other accrued expenses         518,000 2,570,000  
Net cash and cash collateral posted to counterparties used in operating activities         (294,935,000) (112,313,000)  
Cash Flows From Investing Activities:              
Purchases of Agency Securities         (5,317,832,000) (7,971,429,000)  
Principal repayments of Agency Securities         922,148,000 1,036,135,000  
Principal repayments of Credit Risk and Non-Agency Securities         45,766,000 29,186,000  
Proceeds from sales of Agency Securities         10,917,211,000 2,230,377,000  
Disbursements on reverse repurchase agreements         (858,156,000) 0  
Receipts from reverse repurchase agreements         858,156,000 0  
Decrease in cash collateral posted by counterparties         (10,858,000) (71,903,000)  
Net cash and cash collateral posted to counterparties provided by (used in) investing activities         7,228,837,000 (4,638,317,000)  
Cash Flows From Financing Activities:              
Issuance of common stock, net of expenses         48,886,000 321,860,000  
Proceeds from repurchase agreements         62,518,438,000 143,170,110,000  
Principal repayments on repurchase agreements         (69,362,190,000) (138,528,667,000)  
Common stock dividends paid         (55,843,000) (94,300,000)  
Common stock repurchased, net         (777,000) (16,965,000)  
Net cash and cash collateral posted to counterparties provided by (used in) financing activities         (6,939,440,000) 4,805,306,000  
Net Increase (decrease) in cash and cash collateral posted to counterparties         (5,538,000) 54,676,000  
Cash and cash collateral posted to counterparties - beginning of period   $ 273,166,000   $ 232,199,000 273,166,000 232,199,000 $ 232,199,000
Cash and cash collateral posted to counterparties - end of period 267,628,000   286,875,000   267,628,000 286,875,000 $ 273,166,000
Supplemental Disclosure:              
Cash paid during the period for interest         178,719,000 342,374,000  
Non-Cash Investing Activities:              
Payable for unsettled purchases         (518,552,000) 0  
Net unrealized gain (loss) on available for sale Agency Securities         (17,486,000) 451,173,000  
Non-Cash Financing Activities:              
Amounts receivable for issuance of Preferred B stock         0 753,000  
Series A Preferred Stock              
Cash Flows From Financing Activities:              
Redemption of Preferred stock, net of expenses         0 (54,514,000)  
Preferred stock dividends paid         0 (2,249,000)  
Series B Preferred Stock              
Cash Flows From Financing Activities:              
Redemption of Preferred stock, net of expenses         (209,583,000) 0  
Issuance of Preferred stock, net of expenses         0 19,725,000  
Preferred stock dividends paid         (1,375,000) (9,694,000)  
Series C Preferred Stock              
Cash Flows From Financing Activities:              
Issuance of Preferred stock, net of expenses         129,096,000 0  
Preferred stock dividends paid         (6,092,000) 0  
Credit Risk and Non-Agency Securities              
Adjustments to reconcile net loss to net cash and cash collateral posted to counterparties used in operating activities:              
Amortization (accretion)         (2,849,000) (2,266,000)  
Gain (loss) on securities 6,633,000   8,842,000   189,555,000 26,045,000  
Cash Flows From Investing Activities:              
Purchases of Agency Securities, Credit Risk and Non-Agency Securities, and Interest Only Securities         (237,928,000) (10,537,000)  
Proceeds from sales of held-for-investment securities         889,057,000 0  
Interest-Only Securities              
Adjustments to reconcile net loss to net cash and cash collateral posted to counterparties used in operating activities:              
Amortization (accretion)         0 1,922,000  
Gain (loss) on securities 0   0   0 (123,000)  
Cash Flows From Investing Activities:              
Proceeds from sales of held-for-investment securities         0 18,822,000  
U.S. Treasuries              
Adjustments to reconcile net loss to net cash and cash collateral posted to counterparties used in operating activities:              
Amortization (accretion)         84,000 (361,000)  
Gain (loss) on securities $ 0   $ 736,000   (21,771,000) (2,024,000)  
Cash Flows From Investing Activities:              
Purchases of Agency Securities, Credit Risk and Non-Agency Securities, and Interest Only Securities         (4,621,776,000) (1,685,058,000)  
Proceeds from sales of held-for-investment securities         $ 4,643,049,000 $ 1,786,090,000