Derivative Instruments and Hedging Activities (Summary of the Notional Amount and Fair Value of Derivatives) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2020	May 01, 2020	Dec. 22, 2019	Jun. 27, 2019	Sep. 21, 2018
Fair value of derivative instruments
Total fair value derivatives, net	$ (40,135,000)	$ (56,930,000)
Construction Loans | 8770 New Trails
Fair value of derivative instruments
Face amount					$ 35,500,000
$700 Million Loan Maturity September 2023 | Term loan
Fair value of derivative instruments
Face amount		615,000,000.0				$ 615,000,000.0
Derivative instruments not designated as hedging instruments
Fair value of derivative instruments
Total fair value derivative assets	0	1,000
Derivative instruments designated as hedging instruments
Fair value of derivative instruments
Total fair value derivative liabilities	(40,135,000)	(56,931,000)
Interest rate cap | Derivative instruments designated as hedging instruments
Fair value of derivative instruments
Notional amount, asset	230,000,000.0
Fixed Interest Rate 2.50%, Maturing December 2019 | Interest rate cap | Derivative instruments not designated as hedging instruments | Prepaid expenses and other assets, net
Fair value of derivative instruments
Notional amount, asset				$ 230,000,000
Fixed interest rate				2.50%
Total fair value derivative assets	0	0
Interest income	200,000
Fixed Interest Rate 5.0% Maturing August 2022 | Interest rate cap | Derivative instruments not designated as hedging instruments | Prepaid expenses and other assets, net
Fair value of derivative instruments
Notional amount, asset		$ 75,000,000
Fixed interest rate		5.00%
Total fair value derivative assets	0	$ 1,000
Fixed Interest Rate 2.00% To 3.00%, Maturing May 2020 | Interest rate collar | Derivative instruments designated as hedging instruments | Accounts payable and accrued expenses
Fair value of derivative instruments
Notional amount, liability	193,967,000		$ 194,000,000.0
Total fair value derivative liabilities	$ (182,000)	0
Fixed Interest Rate 2.00% To 3.00%, Maturing May 2020 | Interest rate collar | Derivative instruments designated as hedging instruments | Accounts payable and accrued expenses | Minimum
Fair value of derivative instruments
Fixed interest rate	2.00%
Fixed Interest Rate 2.00% To 3.00%, Maturing May 2020 | Interest rate collar | Derivative instruments designated as hedging instruments | Accounts payable and accrued expenses | Maximum
Fair value of derivative instruments
Fixed interest rate	3.00%
Fixed Interest Rate 2.25% To 3.25%, Maturing May 2021 | Interest rate collar | Derivative instruments designated as hedging instruments | Prepaid expenses and other assets, net
Fair value of derivative instruments
Notional amount, liability		354,217,000
Total fair value derivative liabilities	$ (2,074,000)	$ 0
Fixed Interest Rate 2.25% To 3.25%, Maturing May 2021 | Interest rate collar | Derivative instruments designated as hedging instruments | Prepaid expenses and other assets, net | Minimum
Fair value of derivative instruments
Fixed interest rate		2.25%
Fixed Interest Rate 2.25% To 3.25%, Maturing May 2021 | Interest rate collar | Derivative instruments designated as hedging instruments | Prepaid expenses and other assets, net | Maximum
Fair value of derivative instruments
Fixed interest rate		3.25%
Fixed Interest Rate 2.75% To 3.50%, Maturing April 2022 | Interest rate collar | Derivative instruments designated as hedging instruments | Prepaid expenses and other assets, net
Fair value of derivative instruments
Notional amount, liability		$ 381,404,000
Total fair value derivative liabilities	(4,578,000)	$ 0
Fixed Interest Rate 2.75% To 3.50%, Maturing April 2022 | Interest rate collar | Derivative instruments designated as hedging instruments | Prepaid expenses and other assets, net | Minimum
Fair value of derivative instruments
Fixed interest rate		2.75%
Fixed Interest Rate 2.75% To 3.50%, Maturing April 2022 | Interest rate collar | Derivative instruments designated as hedging instruments | Prepaid expenses and other assets, net | Maximum
Fair value of derivative instruments
Fixed interest rate		3.50%
Fixed Interest Rate 2.96%, Maturing September 2023 | Interest rate swap | Derivative instruments designated as hedging instruments | Accounts payable and accrued expenses
Fair value of derivative instruments
Notional amount, liability		$ 615,000,000
Fixed interest rate		2.96%
Total fair value derivative liabilities	(31,187,000)	$ (50,918,000)
Fixed Interest Rate 4.89% Maturing January 2032 | Interest rate swap | Derivative instruments designated as hedging instruments | Accounts payable and accrued expenses
Fair value of derivative instruments
Notional amount, liability		$ 1,810,000
Fixed interest rate		4.89%
Total fair value derivative liabilities	$ (2,114,000)	$ (6,013,000)