Shared-based Compensation (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	$ 830	$ 1,258	$ 1,635	$ 2,215
Weighted-average remaining contractual life of options oustanding			8 years 2 months 9 days
Aggregate intrinsic value of options outstanding	15,700		$ 15,700
Weighted-average remaining contractual life of options exercisable			7 years 7 days
Aggregate intrinsic value of options exercisable	7,900		$ 7,900
Proceeds from stock options exercised			62	568
Intrinsic value of options exercised			1,400	325
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total income tax benefit recognized related to restricted stock awards	546	449	1,481	1,089
Employee service share based compensation nonvested awards total compensation cost not yet recognized	2,300		$ 2,300
Compensation cost recognition weighted average period			1 year 10 months 6 days
Award vesting period			5 years
Share-based compensation arrangement, fair value of awards vested			$ 7,400	$ 2,500
Shares vested in period			85,000	58,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share based compensation nonvested awards total compensation cost not yet recognized	1,000		$ 1,000
Compensation cost recognition weighted average period			3 years 6 months 18 days
Award vesting period			3 years
Share-based compensation, shares authorized under stock option plans, exercise price range, lower range limit			$ 26.52
Share-based compensation arrangement by share-based payment award, options, grants in period, weighted average grant date fair value			18.08	$ 13.73
Share-based compensation, shares authorized under stock option plans, exercise price range, upper range limit			$ 157.30
Proceeds from stock options exercised			$ 62	$ 568
Share based compensation arrangement by share based payment award options expiration term			10 years
Options and Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation	600	961	$ 1,180	1,694
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee service share based compensation nonvested awards total compensation cost not yet recognized	1,400		$ 1,400
Compensation cost recognition weighted average period			1 year 6 months
Share based compensation expense	$ 79	146	$ 152	$ 218
Share-based compensation arrangement shares to be issued	273,300		273,300
Shares vested in period			63,300	113,395
Performance Shares [Member] \| Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation expense	$ 151	$ 303	$ 151	$ 303
Share-based compensation arrangement shares to be issued	210,000	210,000	210,000	210,000

Expiration term for options outstanding during the reporting period.

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- Definition

The compensation expense recognized during the period pertaining to the deferred compensation arrangement.

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Unrecognized cost of unvested share-based compensation awards.

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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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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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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 equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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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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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

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

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- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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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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