Mortgage Notes Payable - Schedule of mortgage notes payable (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 4,377,787	$ 4,392,599
Chandler Fashion Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 255,266	255,174
Effective Interest Rate	4.18%
Monthly Debt Service	$ 875
Danbury Fair Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 191,499	194,718
Effective Interest Rate	5.53%
Monthly Debt Service	$ 1,538
Fashion Outlets of Chicago
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 299,153	299,112
Effective Interest Rate	4.61%
Monthly Debt Service	$ 1,145
Fashion Outlets of Niagara Falls USA
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 104,727	106,398
Effective Interest Rate	4.89%
Monthly Debt Service	$ 727
Freehold Raceway Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 398,462	398,379
Effective Interest Rate	3.94%
Monthly Debt Service	$ 1,300
Fresno Fashion Fair
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 323,758	323,659
Effective Interest Rate	3.67%
Monthly Debt Service	$ 971
Green Acres Commons
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 129,387	$ 128,926
Effective Interest Rate	2.88%	4.40%
Monthly Debt Service	$ 252
Green Acres Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 274,193	$ 277,747
Effective Interest Rate	3.61%
Monthly Debt Service	$ 1,447
Kings Plaza Shopping Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 535,156	535,097
Effective Interest Rate	3.71%
Monthly Debt Service	$ 1,629
Oaks, The
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 185,048	187,142
Effective Interest Rate	4.14%
Monthly Debt Service	$ 1,064
Pacific View
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 116,572	118,202
Effective Interest Rate	4.08%
Monthly Debt Service	$ 668
Queens Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 600,000	600,000
Effective Interest Rate	3.49%
Monthly Debt Service	$ 1,744
Santa Monica Place
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 298,192	297,817
Effective Interest Rate	1.79%
Monthly Debt Service	$ 384
SanTan Village Regional Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 219,186	219,140
Effective Interest Rate	4.34%
Monthly Debt Service	$ 788
Towne Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 20,052	20,284
Effective Interest Rate	4.48%
Monthly Debt Service	$ 117
Tucson La Encantada
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 62,962	63,682
Effective Interest Rate	4.23%
Monthly Debt Service	$ 368
Victor Valley, Mall of
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 114,762	114,733
Effective Interest Rate	4.00%
Monthly Debt Service	$ 380
Vintage Faire Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes	$ 249,412	$ 252,389
Effective Interest Rate	3.55%
Monthly Debt Service	$ 1,256