Related Party Transactions (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Mar. 09, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 01, 2017	Jul. 01, 2016	Jul. 01, 2015	Jul. 01, 2014
Related Party Transaction [Line Items]
Total rental income		$ 298,803	$ 279,498	$ 261,887
Investment in WBW CHP LLC, in percentage		47.70%
WBW CHP LLC
Related Party Transaction [Line Items]
Recovered reimbursements of operating expenses of WBW CHP LLC		$ 1,000	$ 900	$ 1,000
Common Stock
Related Party Transaction [Line Items]
Common shares issued (in shares)		700,000	219,480	1,812,451
Insurance Company of the West, Inc. | Board of Directors Chairman
Related Party Transaction [Line Items]
Total rental income		$ 100	$ 2,200	$ 2,200
Prepaid insurance					$ 200	$ 200	$ 200	$ 400
Percentage of ownership interests classified as noncontrolling		35.20%
Insurance Company of the West, Inc. | Common Stock | Board of Directors Chairman
Related Party Transaction [Line Items]
Percentage of ownership interests classified as noncontrolling		12.20%
Insurance Company of the West, Inc. | Member Units | Board of Directors Chairman
Related Party Transaction [Line Items]
Percentage of ownership interests classified as noncontrolling		23.10%
American Assets, Inc. | Board of Directors Chairman
Related Party Transaction [Line Items]
Total rental income		$ 100
AAI Aviation, Inc. | Board of Directors Chairman
Related Party Transaction [Line Items]
Aviation expense		$ 100	$ 200	$ 200
Explorer Insurance Company | Private Placement | Board of Directors Chairman
Related Party Transaction [Line Items]
Common shares issued (in shares)	200,000
Share price (in dollars per share)	$ 40.54
Gross proceeds from issuance of common stock	$ 8,100