Loans Held for Investment and the Allowance for Credit Losses - Schedule of Collections of Scheduled Interest (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Collection Of Scheduled Interest [Abstract]
Scheduled interest, including PIK	99.30%	99.40%	100.00%	99.30%	100.00%
PIK interest as a percentage of scheduled interest	1.10%	1.20%	1.00%	1.10%	0.40%