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Quarterly Financial Data (Detail) - USD ($)
$ / shares in Units, $ 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
Income Statement [Abstract]                      
Total revenue $ 42,447 $ 40,066 $ 39,561 $ 39,142 $ 38,031 $ 38,364 $ 38,327 $ 38,666 $ 161,216 $ 153,388 $ 109,576
Net income (loss) 6,519 1,156 19,521 4,245 (42,706) 2,407 30,790 (46) $ 31,441 $ (9,555) $ 30,156
Net income (loss) allocable to common shares $ 6,293 $ 1,097 $ 18,739 $ 4,077 $ (40,980) $ 2,267 $ 28,987 $ (75)      
Total earnings (loss) per share—Basic $ 0.08 [1] $ 0.02 [1] $ 0.27 [1] $ 0.06 [1] $ (0.61) [1] $ 0.05 [1] $ 0.61 [1]   $ 0.41 $ (0.19) $ 0.78
Total earnings (loss) per share—Diluted $ 0.08 [1] $ 0.02 [1] $ 0.27 [1] $ 0.06 [1] $ (0.61) [1] $ 0.05 [1] $ 0.61 [1]   $ 0.41 $ (0.19) $ 0.78
[1] The summation of quarterly per share amounts do not equal the full year amounts.