Loans Held for Investment and the Allowance for Credit Losses - Additional Information (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2021 USD ($) Rating Loan LoanModification	Mar. 31, 2021 USD ($) Loan	Jun. 30, 2020 USD ($) Rating	Jun. 30, 2021 USD ($) Rating Loan LoanModification	Jun. 30, 2020 USD ($) Rating	Dec. 31, 2020 USD ($) Rating Loan
Accounts Notes And Loans Receivable [Line Items]
Loans Held for Investment, Net (includes $1,305,947 and $2,259,467, respectively, pledged as collateral under secured credit facilities)	$ 4,773,949,000			$ 4,773,949,000		$ 4,456,460,000
Total loan commitment amount	5,318,297,000			5,318,297,000		4,943,511,000
Unfunded loan commitments	488,916,000			488,916,000		423,487,000
Unamortized loan fees and discounts included in Loans Held for Investment	7,600,000			7,600,000		$ 8,300,000
Accelerated fee component of prepayment fees	$ 0		$ 0
Accelerated fee component of prepayment fees recognized				$ 0	$ 300,000
Weighted Average Risk Rating | Rating	3.1			3.1		3.1
Allowance for credit loss increase (decrease)	$ (3,500,000)			$ (7,500,000)	58,700,000
Allowance for credit loss reserve	55,312,000		$ 58,675,000	55,312,000	58,675,000
Estimate of expected credit losses impacted by loan originations	631,400,000			631,400,000
Estimate of expected credit losses impacted sales				60,700,000
Estimate of expected credit losses impacted repayments				339,300,000
Loans and leases, unpaid principal balance	$ 4,825,890,000			$ 4,825,890,000		$ 4,516,400,000
Number of loan modification | LoanModification	2			7
Aggregate unpaid principal balance of loan modifications outstanding	$ 1,032,900,000			$ 1,032,900,000
Total PIK interest	360,000	$ 816,000		1,200,000
Real Estate
Accounts Notes And Loans Receivable [Line Items]
Allowance for credit loss reserve	$ 10,000,000.0			$ 10,000,000.0		$ 10,000,000.0
Number of loans on non-accrual status | Loan	1			1		1
Amortized cost of loan	$ 31,200,000			$ 31,200,000		$ 31,100,000
Fair market value, discount rate	12.50%			12.50%
Fair market value, terminal capitalization rate	7.50%			7.50%
ASU 2016-13
Accounts Notes And Loans Receivable [Line Items]
Allowance for credit loss increase (decrease)					$ 19,600,000
COVID-19
Accounts Notes And Loans Receivable [Line Items]
Total loan commitment amount	$ 186,900,000			$ 186,900,000
Weighted Average Risk Rating | Rating	3.1		3.1	3.1	3.1	3.1
Allowance for credit loss increase (decrease)					$ 39,100,000
Loans and leases, unpaid principal balance	$ 174,000,000.0			$ 174,000,000.0
Hotel
Accounts Notes And Loans Receivable [Line Items]
Total loan commitment amount	673,043,000			673,043,000		$ 737,293,000
Unfunded loan commitments	6,781,000			6,781,000		9,864,000
Condominiums
Accounts Notes And Loans Receivable [Line Items]
Total loan commitment amount	23,009,000			23,009,000		25,049,000
Office Building
Accounts Notes And Loans Receivable [Line Items]
Total loan commitment amount	2,811,920,000			2,811,920,000		2,756,338,000
Unfunded loan commitments	$ 401,705,000			401,705,000		356,034,000
Rating 2
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan	1
Loans and leases, unpaid principal balance	$ 341,749,000			341,749,000		337,738,000
Moved from Category 3 Risk Rating to Category 2 Risk Rating
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan		1
Moved from Category 4 Risk Rating to Category 3 Risk Rating | Hotel
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan	2
Moved from Category 4 Risk Rating to Category 3 Risk Rating | Condominiums
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan	4
Moved from Category 2 Risk Rating to Category 3 Risk Rating | Office Building
Accounts Notes And Loans Receivable [Line Items]
Number of loans selected for risk rate changes | Loan	1
Nine Loan
Accounts Notes And Loans Receivable [Line Items]
Number of mortgage loans originated or acquired | Loan	9
Total loan commitment amount	$ 752,500,000			752,500,000
Loans and leases receivable unpaid principal balance	597,000,000.0			597,000,000.0
Unfunded loan commitments	155,500,000			$ 155,500,000
Ten Loan
Accounts Notes And Loans Receivable [Line Items]
Number of mortgage loans originated or acquired | Loan				10
Total loan commitment amount	797,800,000			$ 797,800,000
Loans and leases receivable unpaid principal balance	634,500,000			634,500,000
Unfunded loan commitments	163,300,000			163,300,000
One Loan
Accounts Notes And Loans Receivable [Line Items]
Loan commitment principal amount	60,700,000			60,700,000
Loans and leases receivable loans sold	59,500,000
Impairment of investment	1,600,000
Transaction cost	400,000
Accrued Interest and Fees Receivable
Accounts Notes And Loans Receivable [Line Items]
Loans Held for Investment, Net (includes $1,305,947 and $2,259,467, respectively, pledged as collateral under secured credit facilities)	$ 13,900,000			$ 13,900,000		$ 14,000,000.0