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Securities - Summary of Securities Held for Investment and Securities Available for Sale (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Marketable Securities [Abstract]    
Held for investment, at cost (fair value: $2,017 and $2,303, respectively) $ 2,008 $ 2,292
Available for sale, at fair value 460,863 467,703
Total $ 462,871 $ 469,995