Debt (Summary of Debt) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Feb. 28, 2020	Feb. 27, 2020	Dec. 31, 2019
Debt Instrument
Deferred financing costs, net and other	$ (29,000)			$ (38,407)
Long term debt	$ 1,817,981			1,766,712
Mortgages
Debt Instrument
Interest rate, end of period (percent)	3.10%
Long-term debt, gross	$ 5,661,132			5,670,016
Deferred financing costs, net and other	(21,981)			(30,119)
Long term debt	$ 5,639,151			5,639,897
Mortgages | Fixed Rate
Debt Instrument
Interest rate, end of period (percent)	3.69%
Long-term debt, gross	$ 3,886,714			4,601,516
Mortgages | Variable Rate
Debt Instrument
Interest rate, end of period (percent)	1.81%
Long-term debt, gross	$ 1,774,418			1,068,500
Senior Unsecured Notes
Debt Instrument
Interest rate, end of period (percent)	3.50%
Long-term debt, gross	$ 450,000			450,000
Deferred financing costs, net and other	(3,518)			(4,128)
Long term debt	$ 446,482			445,872
Unsecured term loan
Debt Instrument
Interest rate, end of period (percent)	3.70%
Long-term debt, gross	$ 800,000	$ 800,000	$ 750,000	750,000
Deferred financing costs, net and other	(3,501)			(4,160)
Long term debt	$ 796,499			745,840
Unsecured Revolving Credit Facilities
Debt Instrument
Interest rate, end of period (percent)	1.05%
Long term debt	$ 575,000			$ 575,000