Other Assets and Liabilities - Summary of Components of Other Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Held-to-maturity, Past Due [Line Items]
Accrued interest receivable	$ 78,689			$ 66,757
Debt securities held-to-maturity	77,916			77,445
CECL reserve	(280)	$ (122)	$ (889)	(1,723)	$ (2,033)	$ (4,119)	$ (5,122)	$ 0
Debt securities held-to-maturity, net	77,636			75,722
Derivative assets	35,858			522
2021 FL4 CLO restricted cash	25,000			0	$ 0
Loan portfolio payments held by servicer	766			73,224
Prepaid expenses	43			973
Collateral deposited under derivative agreements	0			51,050
Prepaid taxes	0			376
Other	622			1,195
Total	$ 218,614			$ 269,819