Share-Based Compensation Plans - Summary of Share Option Awards (Detail)	12 Months Ended
Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares, Outstanding at January 1, 2017 \| shares	224,379
Shares, Exercised \| shares	(11,086)
Shares, Forfeited or Expired \| shares	(69,000)
Shares, Outstanding at December 31, 2017 \| shares	144,293
Shares, Exercisable at December 31, 2017 \| shares	130,543
Shares, Vested and Expected to Vest at December 31, 2017 \| shares	144,256
Weighted-Average Exercise Price, Outstanding at January 1, 2017 \| $ / shares	$ 14.13
Weighted-Average Exercise Price, Exercised \| $ / shares	6.07
Weighted-Average Exercise Price, Forfeited or Expired \| $ / shares	23.82
Weighted-Average Exercise Price, Outstanding at December 31, 2017 \| $ / shares	10.11
Weighted-Average Exercise Price, Exercisable at December 31, 2017 \| $ / shares	9.74
Weighted-Average Exercise Price, Vested and Expected to Vest at December 31, 2017 \| $ / shares	$ 10.11
Weighted-Average Remaining Contractual Life (In Years), Outstanding at December 31, 2017	3 years
Weighted-Average Remaining Contractual Life (In Years), Exercisable at December 31, 2017	2 years 6 months 21 days
Weighted-Average Remaining Contractual Life (In Years), Vested and Expected to Vest at December 31, 2017	3 years
Aggregate Intrinsic Value, Outstanding at December 31, 2017 \| $	$ 159,083
Aggregate Intrinsic Value, Exercisable at December 31, 2017 \| $	159,083
Aggregate Intrinsic Value, Vested and Expected to Vest at December 31, 2017 \| $	$ 159,083

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of share options (or share units) exercised during the current period.

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