Mortgage Notes Payable - Schedule of mortgage notes payable (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 27, 2019	Dec. 31, 2018
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 4,305,028		$ 4,073,916
Chandler Fashion Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 255,037		199,972
Effective Interest Rate	4.18%	4.18%
Monthly Debt Service	$ 875
Danbury Fair Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 198,489		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	$ 299,072		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	$ 108,033		109,651
Effective Interest Rate	4.89%
Monthly Debt Service	$ 727
Freehold Raceway Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 398,296		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	$ 323,559		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	$ 128,466		$ 128,006
Effective Interest Rate	5.15%		5.06%
Monthly Debt Service	$ 497
Green Acres Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 281,236		$ 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	$ 431,895		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	$ 189,615		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	$ 119,798		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	$ 297,443		297,069
Effective Interest Rate	3.99%
Monthly Debt Service	$ 936
SanTan Village Regional Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 219,034		121,585
Effective Interest Rate	4.34%
Monthly Debt Service	$ 788
Towne Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 20,509		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	$ 64,531		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	$ 114,704		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	$ 255,311		$ 258,207
Effective Interest Rate	3.55%
Monthly Debt Service	$ 1,256