CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Cash Flows [Abstract]
Net income (loss)	$ (889,772)	$ (2,163,091)	$ 54,148
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Amortization of premiums and discounts of investments, net	1,371,178	1,113,273	692,811
Amortization of securitized debt premiums and discounts and deferred financing costs	(11,576)	(11,854)	(3,439)
Depreciation, amortization and other noncash expenses	41,357	31,559	72,364
Net (gains) losses on disposals of investments and other	(661,513)	47,944	1,123,969
Net (gains) losses on investments and derivatives	2,368,879	1,855,025	136,673
Income from unconsolidated joint ventures	7,072	6,893	2,840
Loan loss provision	147,581	16,569	3,496
Payments on purchases of loans held for sale	(147,833)	(250,348)	(227,871)
Proceeds from sales and repayments of loans held for sale	168,716	282,693	97,913
Net receipts (payments) on derivatives	(1,958,131)	(1,939,634)	480,216
Net change in
Other assets	249,778	(39,880)	98,104
Interest receivable	159,320	(85,951)	(19,563)
Interest payable	(285,219)	(94,593)	295,640
Other liabilities	(31,870)	31,838	(185,283)
Net cash provided by (used in) operating activities	527,967	(1,199,557)	2,622,018
Cash flows from investing activities
Payments on purchases of securities	(32,676,856)	(63,465,822)	(44,795,176)
Proceeds from sales of securities	52,639,778	25,606,504	33,256,888
Principal payments on securities	19,571,476	17,199,893	11,488,342
Payments on purchases and origination of loans	(2,257,314)	(4,126,123)	(3,149,224)
Proceeds from sales of loans	624,026	365,787	150,059
Principal payments on loans	2,222,500	3,139,084	2,107,689
Payments on purchases of MSRs	0	0	(381)
Proceeds from sales of MSRs	72,160	0	0
Investments in real estate	(7,450)	(39,144)	(22,722)
Proceeds from sales of real estate	149,600	24,955	0
Proceeds from reverse repurchase agreements	58,800,000	98,339,755	85,318,562
Payments on reverse repurchase agreements	(58,800,000)	(97,689,715)	(85,030,351)
Distributions in excess of cumulative earnings from unconsolidated joint ventures	7,590	3,155	26,228
Cash acquired (paid) in asset acquisition, net	6,264	0	(258,334)
Net cash provided by (used in) investing activities	40,351,774	(20,641,671)	(908,420)
Cash flows from financing activities
Proceeds from repurchase agreements and other secured financing	2,776,331,362	5,470,733,256	5,117,155,986
Principal payments on repurchase agreements and other secured financing	(2,816,805,618)	(5,449,836,013)	(5,116,952,444)
Proceeds from issuances of securitized debt	2,385,374	3,444,055	920,142
Principal repayments on securitized debt	(1,238,962)	(2,031,959)	(1,384,333)
Payment of deferred financing cost	(553)	(12,228)	(1,072)
Net proceeds from stock offerings, direct purchases and dividend reinvestments	1,175	1,829,025	1,532,356
Redemptions of preferred stock	(460,000)	(230,000)	(412,500)
Proceeds from participations issued	38,741	0	0
Net principal receipts (payments) on mortgages payable	(60,980)	(26,202)	(716)
Net contributions (distributions) from (to) noncontrolling interests	7,762	(1,136)	(971)
Net payments on share repurchases	(209,418)	(223,574)	0
Dividends paid	(1,475,650)	(1,689,016)	(1,540,886)
Net cash provided by (used in) financing activities	(41,486,767)	21,956,208	(684,438)
Net (decrease) increase in cash and cash equivalents	(607,026)	114,980	1,029,160
Cash and cash equivalents including cash pledged as collateral, beginning of period	1,850,729	1,735,749	706,589
Cash and cash equivalents including cash pledged as collateral, end of period	1,243,703	1,850,729	1,735,749
Supplemental disclosure of cash flow information
Interest received	3,681,826	4,811,218	3,894,478
Dividends received	4,643	8,395	7,564
Interest paid (excluding interest paid on interest rate swaps)	1,166,977	2,902,644	1,726,887
Net interest received (paid) on interest rate swaps	296,621	(323,028)	(1,894)
Taxes received (paid)	1,515	2,284	(295)
Noncash investing and financing activities
Receivable for unsettled trades	15,912	4,792	68,779
Payable for unsettled trades	884,069	463,387	583,036
Net change in unrealized gains (losses) on available-for-sale securities, net of reclassification adjustment	1,236,144	4,118,056	(853,845)
Dividends declared, not yet paid	307,613	357,527	394,129
Derecognition of assets of consolidated VIEs	1,222,221	0	0
Derecognition of securitized debt of consolidated VIEs	$ 1,141,311	$ 0	$ 0