Notes Receivable and Allowance for Credit Losses - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018
Notes Receivable and Allowance for Losses [Line Items]
Notes receivable	$ 139,712	$ 133,014
Accounting standards update	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Loan reserves	$ 16,295	$ 4,556	$ 4,685
Retained earnings	1,016,635	968,833
Loans and financing receivable	97,809	103,054
Mezzanine & Other Notes Receivable | Impaired Loans
Notes Receivable and Allowance for Losses [Line Items]
Average notes receivable, nonaccrual status	28,900	1,700
Variable Interest Entity, Not Primary Beneficiary | Mezzanine & Other Notes Receivable
Notes Receivable and Allowance for Losses [Line Items]
Loans and financing receivable	121,500	126,500
Variable Interest Entity, Not Primary Beneficiary | Interest Rate Below Market Reduction | Mezzanine & Other Notes Receivable
Notes Receivable and Allowance for Losses [Line Items]
Loans and financing receivable	13,100	16,300
Receivable with Imputed Interest, discount	$ 1,000	1,300
Cumulative Effect, Period of Adoption. Adjusted Balance | Accounting Standards Update 2016-13
Notes Receivable and Allowance for Losses [Line Items]
Loan reserves		12,900
Cumulative Effect, Period of Adoption, Adjustment
Notes Receivable and Allowance for Losses [Line Items]
Loan reserves		8,348
Retained earnings		$ 6,800