Mortgage Notes Payable - Schedule of mortgage notes payable (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 4,162,072	$ 4,073,916
Chandler Fashion Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 199,989	199,972
Effective Interest Rate	3.77%
Monthly Debt Service	$ 625
Danbury Fair Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 200,336	202,158
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	$ 299,055	199,622
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	$ 108,829	109,651
Effective Interest Rate	4.89%
Monthly Debt Service	$ 727
Freehold Raceway
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 398,254	398,212
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	$ 323,509	323,460
Effective Interest Rate	3.67%
Monthly Debt Service	$ 971
Green Acres Commons
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 128,236	$ 128,006
Effective Interest Rate	5.20%	5.06%
Monthly Debt Service	$ 503
Green Acres Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 282,941	$ 284,686
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	$ 434,479	437,120
Effective Interest Rate	3.67%
Monthly Debt Service	$ 2,229
Oaks, The
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 190,832	192,037
Effective Interest Rate	4.14%
Monthly Debt Service	$ 1,064
Pacific View
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 120,583	121,362
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	$ 297,256	297,069
Effective Interest Rate	4.08%
Monthly Debt Service	$ 958
SanTan Village Regional Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 120,776	121,585
Effective Interest Rate	3.14%
Monthly Debt Service	$ 589
Towne Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 20,619	20,733
Effective Interest Rate	4.48%
Monthly Debt Service	$ 117
Tucson La Encantada
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 64,947	65,361
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	$ 114,690	114,675
Effective Interest Rate	4.00%
Monthly Debt Service	$ 380
Vintage Faire Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 256,741	$ 258,207
Effective Interest Rate	3.55%
Monthly Debt Service	$ 1,256