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Income Taxes - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Taxes [Line Items] |  |  |
Securities transactions income tax expense (benefit) | $ 1.4 | $ (21,000) | $ 3,200,000
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% |
Recognition of interest and penalties related to unrecognized tax benefits | $ 0 | $ 0 | $ 0
Earliest Tax Year [Member] |  |  |
Income Taxes [Line Items] |  |  |
Year under examination | 2015 |  |
Latest Tax Year [Member] |  |  |
Income Taxes [Line Items] |  |  |
Year under examination | 2017 |  |

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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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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 + Details
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