Residential Whole Loans (Residential Whole Loans, at Carrying Value - Additional Information) (Details) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	$ 4,251,217,000	$ 2,218,728,000
Purchased Credit Impaired Loans	$ 718,197,000	797,987,000
Weighted Average LTV Ratio	80.00%
Non-QM loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	$ 2,727,006,000	1,354,774,000
Rehabilitation loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	968,867,000	494,576,000
Single-family rental loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	366,362,000	145,327,000
Seasoned performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	188,982,000	$ 224,051,000
Certain Rehabilitation Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for losses	$ 388,000
90 or more
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average LTV Ratio	78.26%	90.24%
Settled Whole Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential whole loans, at carrying value, total or weighted average	$ 4,910,278,000
Unpaid Principal Balance (“UPB”)	$ 5,021,777,000
Weighted Average Coupon	6.03%
Weighted Average Term to Maturity (Months)	274 months
Settled Whole Loans \| Non-QM loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	$ 2,667,482,000
Unpaid Principal Balance (“UPB”)	$ 2,582,746,000
Weighted Average Coupon	6.15%
Weighted Average Term to Maturity (Months)	365 months
Weighted Average LTV Ratio	66.00%
Settled Whole Loans \| Rehabilitation loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	$ 969,255,000
Unpaid Principal Balance (“UPB”)	$ 969,255,000
Weighted Average Coupon	7.32%
Weighted Average Term to Maturity (Months)	8 months
Weighted Average LTV Ratio	65.00%
Settled Whole Loans \| Single-family rental loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	$ 366,362,000
Unpaid Principal Balance (“UPB”)	$ 364,350,000
Weighted Average Coupon	6.27%
Weighted Average Term to Maturity (Months)	325 months
Weighted Average LTV Ratio	69.00%
Settled Whole Loans \| Seasoned performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	$ 188,982,000
Unpaid Principal Balance (“UPB”)	$ 205,243,000
Weighted Average Coupon	4.40%
Weighted Average Term to Maturity (Months)	183 months
Weighted Average LTV Ratio	47.00%
Settled Whole Loans \| Purchased credit impaired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Credit Impaired Loans	$ 718,197,000
Unpaid Principal Balance (“UPB”)	$ 900,183,000
Weighted Average Coupon	4.44%
Weighted Average Term to Maturity (Months)	297 months
Weighted Average LTV Ratio	83.00%
Settled Whole Loans \| Certain Rehabilitation Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchased Performing Loans	$ 250,000,000.0
Weighted Average LTV Ratio	69.00%
Settled Whole Loans \| Current \| Non-QM loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	$ 2,527,231,000
Settled Whole Loans \| Current \| Rehabilitation loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	843,034,000
Settled Whole Loans \| Current \| Single-family rental loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	349,967,000
Settled Whole Loans \| Current \| Seasoned performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	200,643,000
Settled Whole Loans \| 30-59 \| Non-QM loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	29,208,000
Settled Whole Loans \| 30-59 \| Rehabilitation loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	70,130,000
Settled Whole Loans \| 30-59 \| Single-family rental loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	10,974,000
Settled Whole Loans \| 30-59 \| Seasoned performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	2,223,000
Settled Whole Loans \| 60-89 \| Non-QM loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	14,349,000
Settled Whole Loans \| 60-89 \| Rehabilitation loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	16,801,000
Settled Whole Loans \| 60-89 \| Single-family rental loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	2,256,000
Settled Whole Loans \| 60-89 \| Seasoned performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	1,048,000
Settled Whole Loans \| 90 or more \| Non-QM loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	11,958,000
Settled Whole Loans \| 90 or more \| Rehabilitation loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	39,290,000
Settled Whole Loans \| 90 or more \| Single-family rental loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	1,153,000
Settled Whole Loans \| 90 or more \| Seasoned performing loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unpaid Principal Balance (“UPB”)	$ 1,329,000

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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