Mortgage Notes Payable - Schedule of Mortgage Notes Payable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	$ 171,569	$ 176,442
Carrying amount of mortgage notes, other	4,066,511	3,908,976
Chandler Fashion Center
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 199,904	199,833
Effective interest rate (as a percent)	3.77%
Monthly debt service	$ 625
Danbury Fair Mall
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	104,599	107,929
Carrying amount of mortgage notes, other	$ 104,598	107,928
Effective interest rate (as a percent)	5.53%
Monthly debt service	$ 1,538
Fashion Outlets of Chicago
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 199,298	198,966
Effective interest rate (as a percent)	3.02%
Monthly debt service	$ 477
Fashion Outlets of Niagara Falls USA
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 112,770	115,762
Effective interest rate (as a percent)	4.89%
Monthly debt service	$ 727
Freehold Raceway Mall
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	398,050	$ 220,643
Effective interest rate (as a percent)		3.94%
Monthly debt service	1,300
Fresno Fashion Fair
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	$ 0
Carrying amount of mortgage notes, other	$ 323,261	323,062
Effective interest rate (as a percent)	3.67%
Monthly debt service	$ 971
Green Acres Commons
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 107,219	0
Effective interest rate (as a percent)	4.07%
Monthly debt service	$ 322
Green Acres Mall
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 291,366	297,798
Effective interest rate (as a percent)	3.61%
Monthly debt service	$ 1,447
Kings Plaza Shopping Center
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 447,231	456,958
Effective interest rate (as a percent)	3.67%
Monthly debt service	$ 2,229
Northgate Mall
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 0	63,434
Effective interest rate (as a percent)
Monthly debt service
Oaks, The
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 196,732	201,235
Effective interest rate (as a percent)	4.14%
Monthly debt service	$ 1,064
Pacific View
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 124,397	127,311
Effective interest rate (as a percent)	4.08%
Monthly debt service	$ 668
Queens Center
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 600,000	600,000
Effective interest rate (as a percent)	3.49%
Monthly debt service	$ 1,744
Santa Monica Place
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 296,366	219,564
Effective interest rate (as a percent)	3.13%
Monthly debt service	$ 706
SanTan Village Regional Center
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 124,703	127,724
Effective interest rate (as a percent)	3.14%
Monthly debt service	$ 589
Stonewood Center
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 0	99,520
Effective interest rate (as a percent)
Monthly debt service
Towne Mall
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 21,161	21,570
Effective interest rate (as a percent)	4.48%
Monthly debt service	$ 117
Tucson La Encantada
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	66,970	68,513
Carrying amount of mortgage notes, other	$ 0	0
Effective interest rate (as a percent)	4.23%
Monthly debt service	$ 368
Victor Valley, Mall of
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 114,617	114,559
Effective interest rate (as a percent)	4.00%
Monthly debt service	$ 380
Vintage Faire Mall
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 263,818	269,228
Effective interest rate (as a percent)	3.55%
Monthly debt service	$ 1,256
Westside Pavilion
Mortgage loans payable on real estate
Carrying amount of mortgage notes, related party	0	0
Carrying amount of mortgage notes, other	$ 141,020	$ 143,881
Effective interest rate (as a percent)	4.49%
Monthly debt service	$ 783