Mortgage Notes Payable - Schedule of mortgage notes payable (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Jun. 27, 2019	Dec. 31, 2018
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 4,294,633		$ 4,073,916
Chandler Fashion Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 255,086		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	$ 196,616		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	$ 299,092		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	$ 107,229		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	$ 398,337		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	$ 323,609		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	$ 128,696		$ 128,006
Effective interest rate	4.81%		5.06%
Monthly Debt Service	$ 460
Green Acres Mall
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 279,513		$ 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	$ 429,285		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	$ 188,385		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	$ 119,003		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	$ 297,630		297,069
Effective interest rate	3.63%
Monthly Debt Service	$ 845
SanTan Village Regional Center
Mortgage loans payable on real estate
Carrying Amount of Mortgage Notes, Other	$ 219,057		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	$ 20,399		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	$ 64,108		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	$ 114,719		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	$ 253,869		$ 258,207
Effective interest rate	3.55%
Monthly Debt Service	$ 1,256