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Real Estate Dispositions, Held for Sale, and Impairment Charges - Components of Income from Real Estate and Related Assets Held for Sale (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating expense | $ 20,405 | $ 15,867 | $ 14,357 | $ 17,708 | $ 13,733 | $ 15,172 | $ 13,433 | $ 13,257 | $ 68,337 | $ 55,595 | $ 50,965
Asset impairment charges |  |  |  | $ 3,700 |  |  |  |  | 6,835 | 2,016 | 622
Real Estate Held for Sale |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenue |  |  |  |  |  |  |  |  | 872 | 1,121 | 1,121
Operating expense |  |  |  |  |  |  |  |  | 3,538 | 556 | 559
Other income, net |  |  |  |  |  |  |  |  | 0 | (158) | (232)
Income from real estate and related assets sold |  |  |  |  |  |  |  |  | (2,666) | $ 407 | $ 330
Asset impairment charges |  |  |  |  |  |  |  |  | $ 2,800 |  |

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