Loans Held for Investment and the Allowance for Credit Losses - Schedule of Overall Statistics for Loan Portfolio (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) Loan	Dec. 31, 2019 USD ($) Loan
Loans And Leases Receivable Disclosure [Line Items]
Total loan commitment amount	$ 4,943,511	$ 5,628,765
Unpaid principal balance	4,524,725	4,998,176
Unfunded loan commitments	423,487	630,589
Amortized cost	$ 4,516,400	$ 4,980,389
Balance Sheet Portfolio
Loans And Leases Receivable Disclosure [Line Items]
Number of loans | Loan	57	65
Floating rate loans	100.00%	100.00%
Total loan commitment amount	$ 4,943,511	$ 5,628,765
Unpaid principal balance	4,524,725	4,998,176
Unfunded loan commitments	423,487	630,589
Amortized cost	$ 4,516,400	$ 4,980,389
Weighted average credit spread	3.20%	3.50%
Weighted average all-in yield	5.30%	5.70%
Weighted average term to extended maturity (in years)	3 years 1 month 6 days	3 years 9 months 18 days
Total Loan Portfolio
Loans And Leases Receivable Disclosure [Line Items]
Number of loans | Loan	58	66
Floating rate loans	100.00%	100.00%
Total loan commitment amount	$ 5,075,511	$ 5,760,765
Unpaid principal balance	4,524,725	4,998,176
Unfunded loan commitments	423,487	630,589
Amortized cost	$ 4,516,400	$ 4,980,389
Weighted average credit spread	3.20%	3.50%
Weighted average all-in yield	5.30%	5.70%
Weighted average term to extended maturity (in years)	3 years 1 month 6 days	3 years 9 months 18 days