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Investment Securities Available for Sale, at Fair Value - Narrative (Details) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Investments [Line Items]
Contractual maturities (up to)		40 years
Weighted average life of available for sale securities portfolio		7 years 1 month 6 days		5 years
Allowance for credit loss		$ 0
Gross unrealized losses		(13,624,000)		$ (7,591,000)
Other-than-temporary impairment recorded in earnings	$ 0		$ 0
Non-Agency RMBS
Schedule of Investments [Line Items]
Gross unrealized losses		(12,988,000)		(13,000)
CMBS
Schedule of Investments [Line Items]
Gross unrealized losses		$ (636,000)		$ (124,000)

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Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
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Amount of other-than-temporary impairment (OTTI) on investment in debt security, recognized in earnings.

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