Share-Based Compensation - Stock Option Activity under Stock Option Plans (Details) - USD ($)	6 Months Ended
Jun. 30, 2019
Number of Shares
Beginning balance (in shares)	76,300
Exercised (in shares)	(11,200)
Cancelled (in shares)	0
Ending balance (in shares)	65,100
Vested and Expected to Vest (in shares)	65,100
Exercisable (in shares)	65,100
Weighted-Average Exercise Price
Beginning balance (in dollars per share)	$ 12.40
Exercised (in dollars per share)	9.34
Cancelled (in dollars per share)	0
Ending balance (in dollars per share)	12.92
Vested and Expected to Vest (in dollars per share)	12.92
Exercisable (in dollars per share)	$ 12.92
Weighted Average Remaining Contractual Term (in Years)
Outstanding (term)	2 years 9 months 7 days
Vested and Expected to Vest (term)	2 years 9 months 7 days
Exercisable (term)	2 years 9 months 7 days
Aggregate Intrinsic Value
Outstanding	$ 1,625,958
Vested and Expected to Vest	1,625,958
Exercisable	$ 1,625,958

Share-based Compensation Arrangement by Share-based Payment Award, Options, Aggregate Intrinsic Value [Abstract]

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Share-based Compensation Arrangement by Share-based Payment Award, Options, Weighted Average Remaining Contractual Term [Abstract]

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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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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.

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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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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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