Share-Based Compensation - Additional Information (Detail) - USD ($)	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1,930,000	$ 1,982,000
Granted, Number of shares (in shares)	0	0
Stock options exercised (in shares)	34,728
Aggregate intrinsic value of stock options exercised	$ 406,000	$ 318,000
Cash receipts due to exercise of stock options	$ 307,000	313,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in usd per share)	$ 20.55
Granted (in shares)	10,945
Restricted Stock \| 2009 Equity Compensation Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 174,000	$ 190,000
Unrecognized compensation cost	$ 56,000
Unrecognized compensation expense, period of recognition	3 months 18 days
Restricted Stock \| 2009 Equity Compensation Program \| Non-Employee Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in usd per share)	$ 20.55	$ 18.20
Share-based compensation expense	$ 225,000	$ 240,000
Vesting period	1 year	1 year
Shares granted (in shares)	10,945	13,176
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in usd per share)	$ 19.14
Granted (in shares)	153,233
RSUs \| 2009 Equity Compensation Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 1,800,000	$ 1,800,000
Unrecognized compensation cost	$ 2,800,000
Unrecognized compensation expense, period of recognition	1 year 4 months 25 days
Granted (in usd per share)	$ 19.14	$ 19.91
Share-based compensation expense	$ 978,000	$ 866,000
Vesting period	3 years	3 years
Granted (in shares)	151,733	130,523
RSUs \| 2009 Equity Compensation Program \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	2 years
RSUs \| 2009 Equity Compensation Program \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
RSUs \| 2018 Omnibus Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in usd per share)	$ 19.14
Granted (in shares)	1,500
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercised (in shares)	34,728	31,769
Stock Options \| 2009 Equity Compensation Program
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 0

Average annual compensation cost over vesting period.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Net number of non-option equity instruments granted to participants.

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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.

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Gross number of share options (or share units) granted during the period.

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

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