Mortgage Notes Payable - Schedule of mortgage notes payable (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 4,384,680	$ 4,392,599
Chandler Fashion Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 255,221	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, Other	$ 192,792	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, Other	$ 299,132	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, Other	$ 105,559	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, Other	$ 398,420	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, Other	$ 323,708	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, Other	$ 129,157	$ 128,926
Effective interest rate	4.29%	4.40%
Monthly Debt Service	$ 404
Green Acres Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 275,965	$ 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, Other	$ 535,219	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, Other	$ 185,886	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, Other	$ 117,391	118,202
Effective interest rate	4.08%
Monthly Debt Service	$ 668
Queens Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 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, Other	$ 298,005	297,817
Effective interest rate	2.30%
Monthly Debt Service	$ 772
SanTan Village Regional Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 219,163	219,140
Effective interest rate	4.34%
Monthly Debt Service	$ 788
Towne Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 20,168	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, Other	$ 63,251	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, Other	$ 114,748	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, Other	$ 250,895	$ 252,389
Effective interest rate	3.55%
Monthly Debt Service	$ 1,256