Loans Held for Investment and the Allowance for Credit Losses - Summary of Aging Analysis for Loans Held for Investment Portfolio by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	$ 4,749,289	$ 4,516,400
Current
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	4,718,089
Days: 90 or More
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	31,200
Total Loans Past Due
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	31,200
Senior Loans
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	4,714,360	4,484,048
Senior Loans | Current
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	4,683,160
Senior Loans | Days: 90 or More
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	31,200
Senior Loans | Total Loans Past Due
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	31,200
Subordinated and Mezzanine Loans
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	34,929	$ 32,352
Subordinated and Mezzanine Loans | Current
Accounts Notes And Loans Receivable [Line Items]
Total loans past due	$ 34,929