Consolidated Statement of Equity (Unaudited) - 9 months ended Sep. 30, 2015 - USD ($) $ in Thousands	Total	Common Shares	Additional Paid-in Capital	Accumulated Dividends in Excess of Net Income	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests - Unitholders in the Operating Partnership	American Assets Trust, L.P.	American Assets Trust, L.P. Common Shares	Limited Partner American Assets Trust, L.P.	General Partner American Assets Trust, L.P.	Accumulated Other Comprehensive Income (Loss) American Assets Trust, L.P.
Beginning Balance (in shares) at Dec. 31, 2014		43,701,669
Beginning Balance at Dec. 31, 2014	$ 766,576	$ 437	$ 795,065	$ (60,291)	$ 92	$ 31,273
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42,689			30,415		12,274	$ 42,689		$ 12,274	$ 30,415
Common shares issued (in shares)		1,812,451
Common shares issued	72,836	$ 18	72,818
Issuance of restricted stock (in shares)		5,044
Issuance of restricted stock	0	$ 0	0
Forfeiture of restricted stock (in shares)		(26,664)
Forfeiture of restricted stock	0
Conversion of operating partnership units (in shares)		5,741						5,741
Conversion of operating partnership units	0		67			(67)
Dividends declared and paid	(43,585)			(31,100)		(12,485)
Stock-based compensation	2,137		2,137
Shares withheld for employee taxes (in shares)		(149,120)								(149,120)
Shares withheld for employee taxes	(6,573)	$ (1)	(6,572)				(6,573)			$ (6,573)
Other comprehensive loss - change in value of interest rate swap	(1,394)				(999)	(395)	(1,394)				$ (1,394)
Reclassification of amortization of forward-starting swap included in interest expense	(173)				(123)	(50)	(173)				(173)
Ending Balance (in shares) at Sep. 30, 2015		45,349,121
Ending Balance at Sep. 30, 2015	832,513	$ 454	863,515	(60,976)	(1,030)	30,550
Beginning partners' capital account (in shares) at Dec. 31, 2014									17,905,257	43,701,669
Beginning partners' capital account at Dec. 31, 2014							766,576		$ 31,235	$ 735,211	130
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	42,689			$ 30,415		12,274	42,689		$ 12,274	$ 30,415
Contributions from American Assets Trust, Inc. (in shares)										1,812,451
Contributions from American Assets Trust, Inc.							72,836			$ 72,836
Conversion of operating partnership units (in shares)									(5,741)	5,741
Conversion of operating partnership units									$ (67)	$ 67
Issuance of restricted units (in shares)										5,044
Forfeiture of restricted units (in shares)										(26,664)
Distributions							(43,585)		$ (12,485)	$ (31,100)
Stock-based compensation							2,137			$ 2,137
Shares withheld for employee taxes (in shares)		(149,120)								(149,120)
Shares withheld for employee taxes	(6,573)	$ (1)	$ (6,572)				(6,573)			$ (6,573)
Other comprehensive income (loss) - unrealized gain (loss) on swap derivative during the period	(1,394)				(999)	(395)	(1,394)				(1,394)
Reclassification of amortization of forward-starting swap included in interest expense	$ (173)				$ (123)	$ (50)	(173)				(173)
Ending partners' capital account (in shares) at Sep. 30, 2015									17,899,516	45,349,121
Ending partners' capital account at Sep. 30, 2015							$ 832,513		$ 30,957	$ 802,993	$ (1,437)