Consolidated Statement of Equity (Unaudited) - USD ($) $ in Thousands	Total	Interest Expense	Early Extinguishment of Debt	American Assets Trust, L.P.	American Assets Trust, L.P. Interest Expense	American Assets Trust, L.P. Early Extinguishment of Debt	American Assets Trust, L.P. Accumulated Other Comprehensive Income (Loss)	American Assets Trust, L.P. Accumulated Other Comprehensive Income (Loss) Interest Expense	American Assets Trust, L.P. Accumulated Other Comprehensive Income (Loss) Early Extinguishment of Debt	American Assets Trust, L.P. Limited Partner		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	Common Shares American Assets Trust, L.P. General Partner	Additional Paid-in Capital	Accumulated Dividends in Excess of Net Income	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Interest Expense	Accumulated Other Comprehensive Income (Loss) Early Extinguishment of Debt	Noncontrolling Interests - Unitholders in the Operating Partnership	Noncontrolling Interests - Unitholders in the Operating Partnership Interest Expense	Noncontrolling Interests - Unitholders in the Operating Partnership Early Extinguishment of Debt
Beginning Balance (shares) at Dec. 31, 2019														60,068,228
Beginning Balance at Dec. 31, 2019	$ 1,293,672													$ 601				$ 1,452,014	$ (144,378)	$ 5,680			$ (20,245)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,484			$ 15,484						$ 3,312	[1]	$ 12,172	[2]						12,172				3,312
Dividends declared and paid	(22,937)																		(18,020)
Dividends declared and paid (noncontrolling interest)																							(4,917)
Stock-based compensation	1,250																	1,250
Other comprehensive gain - change in value of interest rate swaps	(6,115)			(6,115)			$ (6,115)													(4,816)			(1,299)
Reclassification of amortization of forward-starting swap realized gains included in interest expense		$ (333)			$ (333)			$ (333)													$ (261)			$ (72)
Ending Balance (shares) at Mar. 31, 2020														60,068,228
Ending Balance at Mar. 31, 2020	1,281,021													$ 601				1,453,264	(150,226)	603			(23,221)
Beginning partners' capital account (shares) at Dec. 31, 2019										16,390,548	[1]	60,068,228	[2]
Beginning partners' capital account at Dec. 31, 2019				1,293,672			7,716			$ (22,281)	[1]	$ 1,308,237	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	15,484			15,484						3,312	[1]	12,172	[2]						12,172				3,312
Distributions				(22,937)						$ (4,917)	[1]	(18,020)	[2]
Stock-based compensation				1,250								$ 1,250	[2]
Other comprehensive gain - change in value of interest rate swaps	(6,115)			(6,115)			(6,115)													(4,816)			(1,299)
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(333)			(333)			(333)													(261)			(72)
Ending partners' capital account (shares) at Mar. 31, 2020										16,390,548	[1]	60,068,228	[2]
Ending partners' capital account at Mar. 31, 2020				1,281,021			1,268			$ (23,886)	[1]	$ 1,303,639	[2]
Beginning Balance (shares) at Dec. 31, 2019														60,068,228
Beginning Balance at Dec. 31, 2019	1,293,672													$ 601				1,452,014	(144,378)	5,680			(20,245)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	31,800			31,800
Other comprehensive gain - change in value of interest rate swaps	(4,900)			(4,900)
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(997)	$ 0		(997)	$ 0
Ending Balance (shares) at Sep. 30, 2020														60,282,929
Ending Balance at Sep. 30, 2020	1,265,982													$ 603				1,443,764	(164,462)	1,034			(14,957)
Beginning partners' capital account (shares) at Dec. 31, 2019										16,390,548	[1]	60,068,228	[2]
Beginning partners' capital account at Dec. 31, 2019				1,293,672			7,716			$ (22,281)	[1]	$ 1,308,237	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	31,800			31,800
Other comprehensive gain - change in value of interest rate swaps	(4,900)			(4,900)
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(997)	0		(997)	0
Ending partners' capital account (shares) at Sep. 30, 2020										16,181,537	[1]	60,282,929	[2]
Ending partners' capital account at Sep. 30, 2020				1,265,982			1,819			$ (15,742)	[1]	$ 1,279,905	[2]
Beginning Balance (shares) at Mar. 31, 2020														60,068,228
Beginning Balance at Mar. 31, 2020	1,281,021													$ 601				1,453,264	(150,226)	603			(23,221)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	9,826			9,826						$ 2,101	[1]	$ 7,725	[2]						7,725				2,101
Issuance of restricted stock (shares)												6,008		6,008
Forfeiture of restricted stock (shares)														(318)
Dividends declared and paid	(15,293)																		(12,015)
Dividends declared and paid (noncontrolling interest)																							(3,278)
Stock-based compensation	1,252																	1,252
Other comprehensive gain - change in value of interest rate swaps	14			14			14													10			4
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(332)			(332)			(332)													(261)			(71)
Ending Balance (shares) at Jun. 30, 2020														60,073,918
Ending Balance at Jun. 30, 2020	1,276,488													$ 601				1,454,516	(154,516)	352			(24,465)
Beginning partners' capital account (shares) at Mar. 31, 2020										16,390,548	[1]	60,068,228	[2]
Beginning partners' capital account at Mar. 31, 2020				1,281,021			1,268			$ (23,886)	[1]	$ 1,303,639	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	9,826			9,826						2,101	[1]	$ 7,725	[2]						7,725				2,101
Issuance of restricted stock (shares)												6,008		6,008
Forfeiture of restricted stock (shares)														(318)
Forfeiture of restricted units												(318)
Distributions				(15,293)						$ (3,278)	[1]	$ (12,015)	[2]
Stock-based compensation				1,252								$ 1,252	[2]
Other comprehensive gain - change in value of interest rate swaps	14			14			14													10			4
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(332)			(332)			(332)													(261)			(71)
Ending partners' capital account (shares) at Jun. 30, 2020										16,390,548	[1]	60,073,918	[2]
Ending partners' capital account at Jun. 30, 2020				1,276,488			950			$ (25,063)	[1]	$ 1,300,601	[2]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	6,490			6,490						1,365	[1]	5,125	[2]						5,125				1,365
Dividends declared and paid	(19,116)																		(15,071)
Dividends declared and paid (noncontrolling interest)																							(4,045)
Stock-based compensation	1,251																	1,251
Other comprehensive gain - change in value of interest rate swaps	1,201			1,201			1,201													940			261
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(332)	0		(332)	0		(332)													(261)			(71)
Ending Balance (shares) at Sep. 30, 2020														60,282,929
Ending Balance at Sep. 30, 2020	1,265,982													$ 603				1,443,764	(164,462)	1,034			(14,957)
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	6,490			6,490						1,365	[1]	5,125	[2]						5,125				1,365
Distributions				(19,116)						$ (4,045)	[1]	(15,071)	[2]
Stock-based compensation				1,251								$ 1,251	[2]
Other comprehensive gain - change in value of interest rate swaps	$ 1,201			1,201			1,201													940			261
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(332)	0		(332)	0		(332)													(261)			(71)
Ending partners' capital account (shares) at Sep. 30, 2020										16,181,537	[1]	60,282,929	[2]
Ending partners' capital account at Sep. 30, 2020				1,265,982			1,819			$ (15,742)	[1]	$ 1,279,905	[2]
Increase (Decrease) in Partners' Capital [Roll Forward]
Conversion of operating partnership units (in shares)												209,011		209,011		(209,011)
Conversion of operating partnership units												$ (12,001)		$ 2		$ 12,001		(12,003)		3			11,998
Beginning Balance (shares) at Dec. 31, 2020	60,476,292													60,476,292
Beginning Balance at Dec. 31, 2020	$ 1,253,406													$ 605				1,445,644	(176,560)	1,753			(18,036)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	1,733			1,733						$ 339	[1]	$ 1,394	[2]						1,394				339
Forfeiture of restricted stock (shares)														(4,006)			(4,006)
Dividends declared and paid	(21,463)																		(16,932)
Dividends declared and paid (noncontrolling interest)																							(4,531)
Stock-based compensation	1,484																	1,484
Other comprehensive gain - change in value of interest rate swaps	747			747			747													579			168
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(275)	(193)		(275)	(193)		(275)	$ (193)												(216)	$ (152)		(59)	$ (41)
Ending Balance (shares) at Mar. 31, 2021														60,472,286
Ending Balance at Mar. 31, 2021	1,235,439													$ 605				1,447,128	(192,098)	1,964			(22,160)
Beginning partners' capital account (shares) at Dec. 31, 2020										16,181,537		60,476,292
Beginning partners' capital account at Dec. 31, 2020				1,253,406			2,737			$ (19,020)		$ 1,269,689
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	1,733			1,733						339	[1]	1,394	[2]						1,394				339
Forfeiture of restricted stock (shares)														(4,006)			(4,006)
Distributions				(21,463)						$ (4,531)	[1]	(16,932)	[2]
Stock-based compensation				1,484								$ 1,484	[2]
Other comprehensive gain - change in value of interest rate swaps	$ 747			747			747													579			168
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(275)	(193)		(275)	(193)		(275)	$ (193)												(216)	$ (152)		(59)	$ (41)
Ending partners' capital account (shares) at Mar. 31, 2021										16,181,537		60,472,286
Ending partners' capital account at Mar. 31, 2021				1,235,439			3,016			$ (23,212)		$ 1,255,635
Beginning Balance (shares) at Dec. 31, 2020	60,476,292													60,476,292
Beginning Balance at Dec. 31, 2020	$ 1,253,406													$ 605				1,445,644	(176,560)	1,753			(18,036)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,115			26,115
Other comprehensive gain - change in value of interest rate swaps	$ 1,657			1,657
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(824)	(193)		(824)	(193)
Ending Balance (shares) at Sep. 30, 2021	60,472,065													60,472,065
Ending Balance at Sep. 30, 2021	$ 1,218,691													$ 605				1,450,096	(207,910)	2,248			(26,348)
Beginning partners' capital account (shares) at Dec. 31, 2020										16,181,537		60,476,292
Beginning partners' capital account at Dec. 31, 2020				1,253,406			2,737			$ (19,020)		$ 1,269,689
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	26,115			26,115
Other comprehensive gain - change in value of interest rate swaps	1,657			1,657
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(824)	(193)		(824)	(193)
Ending partners' capital account (shares) at Sep. 30, 2021										16,181,537		60,472,065
Ending partners' capital account at Sep. 30, 2021				1,218,691			3,377			$ (27,477)		$ 1,242,791
Increase (Decrease) in Partners' Capital [Roll Forward]
Conversion of operating partnership units (in shares)															0
Beginning Balance (shares) at Mar. 31, 2021														60,472,286
Beginning Balance at Mar. 31, 2021	1,235,439													$ 605				1,447,128	(192,098)	1,964			(22,160)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	11,487			11,487						$ 2,411		$ 9,076							9,076				2,411
Issuance of restricted stock (shares)												5,184		5,184
Forfeiture of restricted stock (shares)														(2,604)
Dividends declared and paid	(21,464)																		(16,933)
Dividends declared and paid (noncontrolling interest)																							(4,531)
Stock-based compensation	1,484																	1,484
Other comprehensive gain - change in value of interest rate swaps	475			475			475													374			101
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(275)			(275)			(275)													(216)			(59)
Ending Balance (shares) at Jun. 30, 2021														60,474,866
Ending Balance at Jun. 30, 2021	1,227,146													$ 605				1,448,612	(199,955)	2,122			(24,238)
Beginning partners' capital account (shares) at Mar. 31, 2021										16,181,537		60,472,286
Beginning partners' capital account at Mar. 31, 2021				1,235,439			3,016			$ (23,212)		$ 1,255,635
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	11,487			11,487						2,411		$ 9,076							9,076				2,411
Issuance of restricted stock (shares)												5,184		5,184
Forfeiture of restricted stock (shares)														(2,604)
Forfeiture of restricted units												(2,604)
Distributions				(21,464)						$ (4,531)		$ (16,933)
Stock-based compensation				1,484								$ 1,484
Other comprehensive gain - change in value of interest rate swaps	475			475			475													374			101
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(275)			(275)			(275)													(216)			(59)
Ending partners' capital account (shares) at Jun. 30, 2021										16,181,537		60,474,866
Ending partners' capital account at Jun. 30, 2021				1,227,146			3,216			$ (25,332)		$ 1,249,262
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	12,895			12,895						2,709		$ 10,186							10,186				2,709
Forfeiture of restricted stock (shares)												2,801		(2,801)
Dividends declared and paid	(22,995)																		(18,141)
Dividends declared and paid (noncontrolling interest)																							(4,854)
Stock-based compensation	1,484																	1,484
Other comprehensive gain - change in value of interest rate swaps	$ 435			435			435													342			93
Reclassification of amortization of forward-starting swap realized gains included in interest expense		(274)	0		(274)	0		(274)													(216)			(58)
Ending Balance (shares) at Sep. 30, 2021	60,472,065													60,472,065
Ending Balance at Sep. 30, 2021	$ 1,218,691													$ 605				$ 1,450,096	(207,910)	2,248			(26,348)
Increase (Decrease) in Partners' Capital [Roll Forward]
Net income	12,895			12,895						2,709		$ 10,186							$ 10,186				2,709
Forfeiture of restricted stock (shares)												2,801		(2,801)
Distributions				(22,995)						$ (4,854)		$ (18,141)
Stock-based compensation				1,484								$ 1,484
Other comprehensive gain - change in value of interest rate swaps	$ 435			435			435													$ 342			$ 93
Reclassification of amortization of forward-starting swap realized gains included in interest expense		$ (274)	$ 0		$ (274)	$ 0		$ (274)													$ (216)			$ (58)
Ending partners' capital account (shares) at Sep. 30, 2021										16,181,537		60,472,065
Ending partners' capital account at Sep. 30, 2021				$ 1,218,691			$ 3,377			$ (27,477)		$ 1,242,791

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

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