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(Derivative Instruments) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Swaps, at fair value
Derivative [Line Items]
Assets Pledged	$ 0	$ 19,018,000
Notional Amount	$ 0	$ 3,172,000,000
Weighted average fixed-pay rate	0.00%	2.24%
Weighted Average Variable Interest Rate	0.00%	1.81%
Interest expense attributable to Swaps	$ (3,359,000)	$ 927,000	$ 3,780,000
Weighted average Swap rate paid	2.06%	2.28%	2.12%
Weighted average Swap rate received	1.63%	2.24%	1.96%
Swaps, at fair value | Over 3 months to 6 months
Derivative [Line Items]
Notional Amount	$ 0	$ 200,000,000
Weighted average fixed-pay rate	0.00%	2.05%
Weighted Average Variable Interest Rate	0.00%	1.70%
Swaps, at fair value | Over 6 months to 12 months
Derivative [Line Items]
Notional Amount	$ 0	$ 1,430,000,000
Weighted average fixed-pay rate	0.00%	2.30%
Weighted Average Variable Interest Rate	0.00%	1.77%
Swaps, at fair value | Over 12 months to 24 months
Derivative [Line Items]
Notional Amount	$ 0	$ 1,300,000,000
Weighted average fixed-pay rate	0.00%	2.11%
Weighted Average Variable Interest Rate	0.00%	1.86%
Swaps, at fair value | Over 24 months to 36 months
Derivative [Line Items]
Notional Amount	$ 0	$ 20,000,000
Weighted average fixed-pay rate	0.00%	1.38%
Weighted Average Variable Interest Rate	0.00%	1.90%
Swaps, at fair value | Over 36 months to 48 months
Derivative [Line Items]
Notional Amount	$ 0	$ 222,000,000
Weighted average fixed-pay rate	0.00%	2.88%
Weighted Average Variable Interest Rate	0.00%	1.84%
Agency MBS, at fair value | Swaps, at fair value
Derivative [Line Items]
Assets Pledged	$ 0	$ 2,241,000
Restricted cash | Swaps, at fair value
Derivative [Line Items]
Assets Pledged	0	16,777,000
Swaps | Hedging
Derivative [Line Items]
Loss on derivative	0
Swaps | Non-Hedging
Derivative [Line Items]
Loss on derivative	4,300,000	16,500,000	$ 9,600,000
Realized loss	$ 9,400,000	$ 17,700,000

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