Earnings per Share - Reconciliation of Net Loss and Number of Common Shares Used in Computations of Basic Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Numerator - Basic and Diluted
Net loss	$ (70,378)	$ 17,276	$ (34,867)	$ (32,844)	$ (26,483)	$ (37,247)	$ (12,565)	$ (14,714)	$ (38,803)	$ (120,813)	$ (91,009)
Net loss attributable to non-controlling interests									16,465	47,059	39,451
Preferred dividends										(245)
Net loss attributable to Seritage common shareholders	$ (43,456)	$ 10,514	$ (21,219)	$ (19,838)	$ (15,004)	$ (21,102)	$ (7,117)	$ (8,335)	$ (22,338)	$ (73,999)	$ (51,558)
Denominator - Basic and Diluted
Weighted average common shares outstanding									31,386	33,804	31,416
Net loss per share attributable to Class A and Class C common shareholders									$ (0.71)	$ (2.19)	$ (1.64)
Class A Common Shares [Member]
Denominator - Basic and Diluted
Weighted average common shares outstanding									24,707	28,249	25,497
Class C Common Shares [Member]
Denominator - Basic and Diluted
Weighted average common shares outstanding									6,679	5,555	5,919