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Share-Based Compensation - Intrinsic Value (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Share based Compensation Arrangements by Share based Payment Award, Performance Options [Line Items] |  |  |
Intrinsic value of options vested | $ 462,369 | $ 4,843,774 | $ 5,000
Intrinsic value of options exercised | $ 8,025,527 | $ 1,777,476 | $ 1,309,000

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- Definition Schedule of Share based Compensation Arrangements by Share based Payment Award, Performance Options [Line Items] + References No definition available. + Details
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- Definition Share Based Compensation Arrangement by Share Based Payment Award, Options, Vested in Period, Total Intrinsic Value + References No definition available. + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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