Consolidated Statement of Equity (Unaudited) - USD ($) $ in Thousands		Total	American Assets Trust, L.P.	American Assets Trust, L.P. Accumulated Other Comprehensive Income (Loss)	American Assets Trust, L.P. Limited Partner	[1]	American Assets Trust, L.P. General Partner		Common Shares	Common Shares American Assets Trust, L.P.	Common Shares American Assets Trust, L.P. Limited Partner	Additional Paid-in Capital	Accumulated Dividends in Excess of Net Income	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests - Unitholders in the Operating Partnership	Swap	Swap American Assets Trust, L.P.	Swap American Assets Trust, L.P. Accumulated Other Comprehensive Income (Loss)	Swap Accumulated Other Comprehensive Income (Loss)	Swap Noncontrolling Interests - Unitholders in the Operating Partnership	Forward Contracts	Forward Contracts American Assets Trust, L.P.	Forward Contracts American Assets Trust, L.P. Accumulated Other Comprehensive Income (Loss)	Forward Contracts Accumulated Other Comprehensive Income (Loss)	Forward Contracts Noncontrolling Interests - Unitholders in the Operating Partnership
Beginning Balance (shares) at Dec. 31, 2017									47,204,588
Beginning Balance at Dec. 31, 2017		$ 844,144							$ 473			$ 919,066	$ (97,280)	$ 11,451	$ 10,434
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		(691)	$ (691)		$ (166)		$ (525)	[2]					(525)		(166)
Forfeiture of restricted stock (shares)									(1,104)
Dividends declared and paid		(17,388)											(12,745)
Dividends declared and paid (noncontrolling interest)															(4,643)
Stock-based compensation		727										727
Other comprehensive income - change in value of interest rate swaps																$ 2,861	$ 2,861	$ 2,861	$ 2,107	$ 754
Reclassification of amortization of forward-starting swap included in interest expense		(320)	(320)	$ (320)										(234)	(86)
Ending Balance (shares) at Mar. 31, 2018									47,203,484
Ending Balance at Mar. 31, 2018		829,333							$ 473			919,793	(110,550)	13,324	6,293
Beginning partners' capital account (shares) at Dec. 31, 2017					17,194,980		47,204,588	[2]
Beginning partners' capital account at Dec. 31, 2017			844,144	15,750	$ 6,135		$ 822,259	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		(691)	(691)		(166)		$ (525)	[2]					(525)		(166)
Partners' Capital Account, Restricted Units Forfeiture	[2]						(1,104)
Distributions			(17,388)		$ (4,643)		$ (12,745)	[2]
Stock-based compensation			727				$ 727	[2]
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																2,861	2,861	2,861	2,107	754
Reclassification of amortization of forward-starting swap included in interest expense		(320)	(320)	(320)										(234)	(86)
Ending partners' capital account (shares) at Mar. 31, 2018					17,194,980		47,203,484	[2]
Ending partners' capital account at Mar. 31, 2018			829,333	18,291	$ 1,326		$ 809,716	[2]
Beginning Balance (shares) at Dec. 31, 2017									47,204,588
Beginning Balance at Dec. 31, 2017		844,144							$ 473			919,066	(97,280)	11,451	10,434
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		17,993	17,993
Other comprehensive income - change in value of interest rate swaps																					$ 0	$ 0
Reclassification of amortization of forward-starting swap included in interest expense		(959)	(959)
Ending Balance (shares) at Sep. 30, 2018									47,222,121
Ending Balance at Sep. 30, 2018		815,491							$ 473			920,324	(122,293)	13,915	3,072
Beginning partners' capital account (shares) at Dec. 31, 2017					17,194,980		47,204,588	[2]
Beginning partners' capital account at Dec. 31, 2017			844,144	15,750	$ 6,135		$ 822,259	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		17,993	17,993
Units withheld for employee taxes		6	6
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																					0	0
Reclassification of amortization of forward-starting swap included in interest expense		(959)	(959)
Ending partners' capital account (shares) at Sep. 30, 2018					17,177,608		47,222,121	[2]
Ending partners' capital account at Sep. 30, 2018			815,491	19,094	$ (2,107)		$ 798,504	[2]
Beginning Balance (shares) at Mar. 31, 2018									47,203,484
Beginning Balance at Mar. 31, 2018		829,333							$ 473			919,793	(110,550)	13,324	6,293
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		$ 4,413	4,413		$ 1,125		$ 3,288	[2]					3,288		1,125
Issuance of restricted stock (shares)		5,320
Forfeiture of restricted stock (shares)									(2,208)
Conversion of operating partnership units (in shares)									17,372
Conversion of operating partnership units												(916)			916
Dividends declared and paid		$ (17,388)											(12,746)
Dividends declared and paid (noncontrolling interest)															(4,642)
Stock-based compensation		$ 727										727
Shares withheld for employee taxes (shares)		(159)					159
Shares withheld for employee taxes												(6)
Other comprehensive income - change in value of interest rate swaps																875	875	875	643	232
Reclassification of amortization of forward-starting swap included in interest expense		$ (319)	(319)	(319)										(233)	(86)
Ending Balance (shares) at Jun. 30, 2018									47,223,809
Ending Balance at Jun. 30, 2018		817,635							$ 473			919,598	(120,008)	13,734	3,838
Beginning partners' capital account (shares) at Mar. 31, 2018					17,194,980		47,203,484	[2]
Beginning partners' capital account at Mar. 31, 2018			829,333	18,291	$ 1,326		$ 809,716	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		$ 4,413	4,413		1,125		$ 3,288	[2]					3,288		1,125
Issuance of restricted units (shares)	[2]						5,320
Partners' Capital Account, Restricted Units Forfeiture	[2]						(2,208)
Conversion of operating partnership units (shares)							17,372	[2]			(17,372)
Conversion of operating partnership units							$ (916)				$ 916
Distributions			(17,388)		$ (4,642)		(12,746)	[2]
Stock-based compensation			727				$ 727	[2]
Units withheld for employee taxes (shares)		(159)					159
Units withheld for employee taxes			(6)				$ (6)
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																875	875	875	643	232
Reclassification of amortization of forward-starting swap included in interest expense		$ (319)	(319)	(319)										(233)	(86)
Ending partners' capital account (shares) at Jun. 30, 2018					17,177,608		47,223,809	[2]
Ending partners' capital account at Jun. 30, 2018			817,635	18,847	$ (1,275)		$ 800,063	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		14,271	14,271		3,806		10,465	[2]					10,465		3,806
Forfeiture of restricted stock (shares)									(1,688)
Dividends declared and paid		(17,388)											(12,750)
Dividends declared and paid (noncontrolling interest)															(4,638)
Stock-based compensation		726										726
Other comprehensive income - change in value of interest rate swaps																567	567	567	416	151	0	0
Reclassification of amortization of forward-starting swap included in interest expense		(320)	(320)	(320)										(235)	(85)
Ending Balance (shares) at Sep. 30, 2018									47,222,121
Ending Balance at Sep. 30, 2018		815,491							$ 473			920,324	(122,293)	13,915	3,072
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		14,271	14,271		3,806		$ 10,465	[2]					10,465		3,806
Partners' Capital Account, Restricted Units Forfeiture	[2]						(1,688)
Distributions			(17,388)		$ (4,638)		$ (12,750)	[2]
Stock-based compensation			726				$ 726	[2]
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																567	567	567	416	151	0	0
Reclassification of amortization of forward-starting swap included in interest expense		(320)	(320)	(320)										(235)	(85)
Ending partners' capital account (shares) at Sep. 30, 2018					17,177,608		47,222,121	[2]
Ending partners' capital account at Sep. 30, 2018			815,491	19,094	$ (2,107)		$ 798,504	[2]
Beginning Balance (shares) at Dec. 31, 2018									47,335,409
Beginning Balance at Dec. 31, 2018		802,471							$ 474			920,661	(128,778)	10,620	(506)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		15,243	15,243		$ 4,055		$ 11,188	[2]					11,188		4,055
Common shares issued (shares)									162,531
Common shares issued		7,034							$ 1			7,033
Forfeiture of restricted stock (shares)									(11,046)
Forfeiture of restricted stock									$ 0			0
Dividends declared and paid		(18,061)											(13,251)
Dividends declared and paid (noncontrolling interest)															(4,810)
Stock-based compensation		1,098										1,098
Other comprehensive income - change in value of interest rate swaps																(2,132)	(2,132)	(2,132)	(1,561)	(571)
Reclassification of amortization of forward-starting swap included in interest expense		(320)	(320)	(320)										(234)	(86)
Ending Balance (shares) at Mar. 31, 2019									47,486,894
Ending Balance at Mar. 31, 2019		805,333							$ 475			928,792	(130,841)	8,825	(1,918)
Beginning partners' capital account (shares) at Dec. 31, 2018					17,177,608		47,335,409	[2]
Beginning partners' capital account at Dec. 31, 2018			802,471	14,591	$ (4,477)		$ 792,357	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		15,243	15,243		4,055		$ 11,188	[2]					11,188		4,055
Contributions from American Assets Trust, Inc. (in shares)	[2]						162,531
Contributions from American Assets Trust, Inc.			7,034				$ 7,034	[2]
Partners' Capital Account, Restricted Units Forfeiture	[2]						(11,046)
Distributions			(18,061)		$ (4,810)		$ (13,251)	[2]
Stock-based compensation			1,098				$ 1,098	[2]
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																(2,132)	(2,132)	(2,132)	(1,561)	(571)
Reclassification of amortization of forward-starting swap included in interest expense		(320)	(320)	(320)										(234)	(86)
Ending partners' capital account (shares) at Mar. 31, 2019					17,177,608		47,486,894	[2]
Ending partners' capital account at Mar. 31, 2019			805,333	12,139	$ (5,232)		$ 798,426	[2]
Beginning Balance (shares) at Dec. 31, 2018									47,335,409
Beginning Balance at Dec. 31, 2018		802,471							$ 474			920,661	(128,778)	10,620	(506)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		43,703	43,703
Conversion of operating partnership units (in shares)										787,060
Other comprehensive income - change in value of interest rate swaps																					513	513
Reclassification of amortization of forward-starting swap included in interest expense		(972)	(972)
Ending Balance (shares) at Sep. 30, 2019									59,956,972
Ending Balance at Sep. 30, 2019		1,298,224							$ 599			1,450,816	(139,307)	5,162	(19,046)
Beginning partners' capital account (shares) at Dec. 31, 2018					17,177,608		47,335,409	[2]
Beginning partners' capital account at Dec. 31, 2018			802,471	14,591	$ (4,477)		$ 792,357	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		43,703	43,703
Units withheld for employee taxes		0	0
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																					513	513
Reclassification of amortization of forward-starting swap included in interest expense		(972)	(972)
Ending partners' capital account (shares) at Sep. 30, 2019					16,390,548		59,956,972	[2]
Ending partners' capital account at Sep. 30, 2019			1,298,224	7,015	$ (20,899)		$ 1,312,108	[2]
Beginning Balance (shares) at Mar. 31, 2019									47,486,894
Beginning Balance at Mar. 31, 2019		805,333							$ 475			928,792	(130,841)	8,825	(1,918)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		11,941	11,941		$ 2,933		$ 9,008	[2]					9,008		2,933
Common shares issued (shares)									11,444,382
Common shares issued		$ 495,980							$ 114			495,866
Issuance of restricted stock (shares)		4,412
Conversion of operating partnership units (in shares)									787,060
Conversion of operating partnership units									$ 8			12,979		148	(13,135)
Dividends declared and paid		$ (18,253)											(13,664)
Dividends declared and paid (noncontrolling interest)															(4,589)
Stock-based compensation		1,131										1,131
Other comprehensive income - change in value of interest rate swaps																(3,891)	(3,891)	(3,891)	(2,857)	(1,034)
Reclassification of amortization of forward-starting swap included in interest expense		(319)	(319)	(319)										(240)	(79)
Ending Balance (shares) at Jun. 30, 2019									59,722,748
Ending Balance at Jun. 30, 2019		1,291,922							$ 597			1,438,768	(135,497)	5,876	(17,822)
Beginning partners' capital account (shares) at Mar. 31, 2019					17,177,608		47,486,894	[2]
Beginning partners' capital account at Mar. 31, 2019			805,333	12,139	$ (5,232)		$ 798,426	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		11,941	11,941		$ 2,933		$ 9,008	[2]					9,008		2,933
Contributions from American Assets Trust, Inc. (in shares)	[2]						11,444,382
Contributions from American Assets Trust, Inc.			495,980				$ 495,980	[2]
Issuance of restricted units (shares)	[2]						4,412
Conversion of operating partnership units (shares)					(787,060)		787,060	[2]
Conversion of operating partnership units					$ (12,987)		$ 12,987	[2]
Distributions			(18,253)		$ (4,589)		(13,664)	[2]
Stock-based compensation			1,131				$ 1,131	[2]
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																(3,891)	(3,891)	(3,891)	(2,857)	(1,034)
Reclassification of amortization of forward-starting swap included in interest expense		(319)	(319)	(319)										(240)	(79)
Ending partners' capital account (shares) at Jun. 30, 2019					16,390,548		59,722,748	[2]
Ending partners' capital account at Jun. 30, 2019			1,291,922	7,929	$ (19,875)		$ 1,303,868	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		16,519	16,519		3,565		12,954	[2]					12,954		3,565
Common shares issued (shares)									234,814
Common shares issued		10,920							$ 2			10,918
Forfeiture of restricted stock (shares)									(590)
Forfeiture of restricted stock									$ 0			0
Conversion of operating partnership units (in shares)										0
Dividends declared and paid		(21,353)											(16,764)
Dividends declared and paid (noncontrolling interest)															(4,589)
Stock-based compensation		1,130										1,130
Other comprehensive income - change in value of interest rate swaps																(1,094)	(1,094)	(1,094)	(841)	(253)	513	513	$ 513	$ 388	$ 125
Reclassification of amortization of forward-starting swap included in interest expense		(333)	(333)	(333)										(261)	(72)
Ending Balance (shares) at Sep. 30, 2019									59,956,972
Ending Balance at Sep. 30, 2019		1,298,224							$ 599			$ 1,450,816	(139,307)	5,162	(19,046)
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income		16,519	16,519		3,565		$ 12,954	[2]					$ 12,954		3,565
Contributions from American Assets Trust, Inc. (in shares)	[2]						234,814
Contributions from American Assets Trust, Inc.			10,920				$ 10,920	[2]
Issuance of restricted units (shares)	[2]						0
Partners' Capital Account, Restricted Units Forfeiture	[2]						(590)
Distributions			(21,353)		$ (4,589)		$ (16,764)	[2]
Stock-based compensation			1,130				$ 1,130	[2]
Other comprehensive (loss) income - unrealized (loss) income on swap derivative during the period																$ (1,094)	$ (1,094)	$ (1,094)	$ (841)	$ (253)	$ 513	$ 513	$ 513	$ 388	$ 125
Reclassification of amortization of forward-starting swap included in interest expense		$ (333)	(333)	(333)										$ (261)	$ (72)
Ending partners' capital account (shares) at Sep. 30, 2019					16,390,548		59,956,972	[2]
Ending partners' capital account at Sep. 30, 2019			$ 1,298,224	$ 7,015	$ (20,899)		$ 1,312,108	[2]

[1] Consists of limited partnership interests held by third parties.

[2] Consists of general partnership interests held by American Assets Trust, Inc.