Debt and Preferred Equity Investments - Narrative (Details)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) investment	Mar. 31, 2021 segment	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment investment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) investment	Dec. 31, 2019 USD ($)	Aug. 31, 2018 receivable
Preferred equity investment
Number of investments, nonperforming | investment	1			1		1
Debt and preferred equity investments	$ 1,052,110,000			$ 1,052,110,000		$ 1,076,542,000
Recorded investment, past due	1,052,110,000			1,052,110,000		1,076,542,000
Recorded investment, nonaccrual status	27,700,000			$ 27,700,000		27,700,000
Number of portfolio segments of financial receivables (segment) | segment		1		1
Additional amount of financing receivables included in other assets	50,300,000			$ 50,300,000		66,200,000
Current period provision for loan loss	0		$ 0	0	$ 6,300,000	20,693,000
Allowance for loan and lease losses, real estate	6,630,000			6,630,000		13,213,000	$ 1,750,000
3 - High Risk Assets - Loss more likely than not
Preferred equity investment
Recorded investment, past due	6,890,000			6,890,000		16,340,000
Recorded investment, nonaccrual status	2,500,000			2,500,000
Number of receivables put on nonaccrual | receivable								1
Financing Receivables, Equal to Greater than 90 Days Past Due
Preferred equity investment
Recorded investment, past due	0			0		0
Cumulative Effect, Period of Adoption, Adjustment
Preferred equity investment
Allowance for loan and lease losses, real estate						0	$ 27,803,000
Total floating rate
Preferred equity investment
Debt and preferred equity investments	281,495,000			281,495,000		352,460,000
Total floating rate | Mortgage/Mezzanine Loan Due March 2020
Preferred equity investment
Debt and preferred equity investments	$ 6,800,000			$ 6,800,000		$ 6,800,000