Loans Held for Investment and the Allowance for Credit Losses - Summary of Amortized Cost by Origination Year Grouped by Risk Rating (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, One	$ 631,871	$ 247,770
Amortized cost basis of loans by origination year, Two	253,369	2,171,469
Amortized cost basis of loans by origination year, Three	1,887,743	1,515,229
Amortized cost basis of loans by origination year, Four	1,476,077	556,883
Amortized cost basis of loans by origination year, Five	553,821	25,049
Amortized cost basis of loans by origination year, Prior	23,009
Amortized cost basis of loans by origination year, Total	4,825,890	4,516,400
Rating 2
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Total	341,749	337,738
Rating 3
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Total	3,872,528	3,340,663
Rating 4
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Total	580,413	806,893
Rating 5
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Total	31,200	31,106
Senior Loans
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, One	631,871	247,770
Amortized cost basis of loans by origination year, Two	253,369	2,139,117
Amortized cost basis of loans by origination year, Three	1,853,583	1,515,229
Amortized cost basis of loans by origination year, Four	1,476,077	556,883
Amortized cost basis of loans by origination year, Five	553,821	25,049
Amortized cost basis of loans by origination year, Prior	23,009
Amortized cost basis of loans by origination year, Total	4,791,730	4,484,048
Senior Loans | Rating 2
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, One	33,345
Amortized cost basis of loans by origination year, Three		337,738
Amortized cost basis of loans by origination year, Four	308,404
Amortized cost basis of loans by origination year, Total	341,749	337,738
Senior Loans | Rating 3
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, One	598,526	247,770
Amortized cost basis of loans by origination year, Two	253,369	1,705,783
Amortized cost basis of loans by origination year, Three	1,536,789	1,099,503
Amortized cost basis of loans by origination year, Four	1,089,536	255,255
Amortized cost basis of loans by origination year, Five	337,139
Amortized cost basis of loans by origination year, Prior	23,009
Amortized cost basis of loans by origination year, Total	3,838,368	3,308,311
Senior Loans | Rating 4
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Two		433,334
Amortized cost basis of loans by origination year, Three	316,794	46,882
Amortized cost basis of loans by origination year, Four	46,937	301,628
Amortized cost basis of loans by origination year, Five	216,682	25,049
Amortized cost basis of loans by origination year, Total	580,413	806,893
Senior Loans | Rating 5
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Three		31,106
Amortized cost basis of loans by origination year, Four	31,200
Amortized cost basis of loans by origination year, Total	31,200	31,106
Subordinated and Mezzanine Loans
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Two		32,352
Amortized cost basis of loans by origination year, Three	34,160
Amortized cost basis of loans by origination year, Total	34,160	32,352
Subordinated and Mezzanine Loans | Rating 3
Accounts Notes And Loans Receivable [Line Items]
Amortized cost basis of loans by origination year, Two		32,352
Amortized cost basis of loans by origination year, Three	34,160
Amortized cost basis of loans by origination year, Total	$ 34,160	$ 32,352