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Income Taxes - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Taxes [Line Items] |  |  |
Securities transactions income tax expense (benefit) | $ 10,000 | $ 1.4 | $ (21,000)
Change in income tax expense (benefit) due to write down of net deferred tax asset resulting from federal tax rate change |  | $ 4,300,000 |
Federal tax rate, percentage | 21.00% | 35.00% | 35.00%
Recognition of interest and penalties related to unrecognized tax benefits | $ 0 | $ 0 | $ 0
Earliest Tax Year [Member] |  |  |
Income Taxes [Line Items] |  |  |
Year under examination | 2016 | 2013 |
Latest Tax Year [Member] |  |  |
Income Taxes [Line Items] |  |  |
Year under examination | 2018 | 2015 |

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- Definition Tax cuts and jobs act of 2017 change in tax rate income tax expense benefit + References No definition available. + Details
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