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Quarterly Financial Data (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Statement [Abstract]                      
Total revenue $ 49,809 $ 48,779 $ 46,889 $ 45,755 $ 42,447 $ 40,066 $ 39,561 $ 39,142 $ 191,232 $ 161,216 $ 153,388
Net income (loss) 14,729 4,836 3,545 3,500 6,519 1,156 19,521 4,245 $ 26,610 $ 31,441 $ (9,555)
Net income (loss) allocable to common shares $ 14,580 $ 4,787 $ 3,509 $ 3,412 $ 6,293 $ 1,097 $ 18,739 $ 4,077      
Total earnings (loss) per share—Basic $ 0.16 [1] $ 0.05 [1] $ 0.04 [1] $ 0.04 [1] $ 0.08 [1] $ 0.02 [1] $ 0.27 [1] $ 0.06 [1] $ 0.30 $ 0.41 $ (0.19)
Total earnings (loss) per share—Diluted $ 0.16 [1] $ 0.05 [1] $ 0.04 [1] $ 0.04 [1] $ 0.08 [1] $ 0.02 [1] $ 0.27 [1] $ 0.06 [1] $ 0.30 $ 0.41 $ (0.19)
[1] The summation of quarterly per share amounts may not equal the full year amounts due to rounding.