Derivative Instruments - Narrative (Details) $ in Millions	3 Months Ended
Mar. 31, 2020 USD ($)
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Net premiums received on derivative instrument assets	$ 0.7

Net Premiums Paid (Received) On Derivative Instrument Assets

No definition available.

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Namespace Prefix:	bsprt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration