Loans and Investments - Purchased Credit Deterioration and Troubled Debt Restructurings (Details) $ in Thousands	1 Months Ended			9 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Jan. 31, 2021	Aug. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) item loan	Sep. 30, 2020 USD ($) item	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)
Financing Receivable, Recorded Investment, Past Due [Line Items]
Purchase of loan				$ 5,174,313	$ 1,425,051
Allowance for credit losses	$ 138,447			$ 131,534	$ 146,745	$ 148,329		$ 152,811
Proceeds from full satisfaction of loans	95,000
Reversal of allowance for credit losses	7,500
Interest income from loans	3,500
Troubled debt restructurings, number | item				2
Other loans modification | item				0	0
Interest reserve				$ 89,200		$ 78,300
Number of loans covered under interest reserve | loan				297		186
Aggregate UPB covered under interest reserve				$ 5,270,000		$ 3,600,000
Hotel
Financing Receivable, Recorded Investment, Past Due [Line Items]
Allowance for credit losses	225			16	$ 7,539	7,759		7,672
Hotel | Forbearance agreement
Financing Receivable, Recorded Investment, Past Due [Line Items]
Fixed rate of interest (as a percent)		9.50%
Principal reduction			$ (10,000)
Pay rate (as a percent)			1.00%
Hotel | Forbearance agreement | LIBOR
Financing Receivable, Recorded Investment, Past Due [Line Items]
Variable rate, spread (as a percent)			3.00%
LIBOR Floor rate			1.50%
Retail
Financing Receivable, Recorded Investment, Past Due [Line Items]
Allowance for credit losses	$ 13,819			13,819	$ 13,941	13,861		$ 13,981
Retail | Loan modification agreement
Financing Receivable, Recorded Investment, Past Due [Line Items]
Maturity period (in years)			3 years
Percentage of interest foregone			2.00
Principal paydowns			$ 6,000
Principal reduction			(8,000)
Additional reserve deposit			$ 4,600
Retail | Loan modification agreement | LIBOR
Financing Receivable, Recorded Investment, Past Due [Line Items]
Variable rate, spread (as a percent)			5.50%
LIBOR Floor rate			6.50%
Bridge loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Principal amount				35,000
Bridge loans | Hotel
Financing Receivable, Recorded Investment, Past Due [Line Items]
Principal amount							$ 110,000
Purchase of loan			$ 60,000				$ 50,000
Allowance for credit losses									$ 7,500
Loan discount			39,900
Total discount received			20,100
Bridge loans | Retail | Loan modification agreement
Financing Receivable, Recorded Investment, Past Due [Line Items]
Principal amount			$ 26,500
Bridge loans | Retail | Loan modification agreement | LIBOR
Financing Receivable, Recorded Investment, Past Due [Line Items]
Variable rate, spread (as a percent)			6.00%
LIBOR Floor rate			2.375%
Mezzanine loans | Retail | Loan modification agreement
Financing Receivable, Recorded Investment, Past Due [Line Items]
Principal amount			$ 6,100
Fixed rate of interest (as a percent)			12.00%
Greater than 90 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans past due 90 days or more still accruing interest				$ 0		$ 0