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Marketable Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Jun. 30, 2018 | Dec. 31, 2017
Investment Holdings [Line Items] |  |  |  |  |  |
Change in fair value of marketable securities (see Note 7) | $ (824) | $ 0 | $ (5,561) | $ 0 |  |
Macerich Interest |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Macerich common shares | 535,265 |  | 535,265 |  |  | 535,265
Economic basis per share | $ 56.05 |  | $ 56.05 |  |  |
Economic cost | $ 30,000 |  | $ 30,000 |  |  |
Fair value | $ 29,595 |  | $ 29,595 |  |  | $ 35,156
Closing share price | $ 55.29 |  | $ 55.29 |  | $ 56.83 | $ 65.68
Change in fair value of marketable securities (see Note 7) | $ (824) |  | $ (5,561) |  |  |

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