Stock-Based Compensation - Schedule of Information Related to Outstanding SOSARS (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercised	(36,000)	(22,000)	(26,000)
Stock Only Stock Appreciation Rights (SOSARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance, Outstanding	2,736,616	2,199,742	1,864,707
Granted	0	743,925	423,590
Exercised	(157,140)	(61,190)	(70,347)
Canceled / forfeited	(17,630)	(145,861)	(18,208)
Ending Balance, Outstanding	2,561,846	2,736,616	2,199,742
Exercisable at December 31	2,011,075	1,740,591	1,504,599
Vested and expected to vest	2,561,846	2,725,611	2,178,708
Beginning Balance, Weighted Average Exercise Price, Outstanding	$ 17.64	$ 17.82	$ 16.20
Weighted Average Exercise Price, Granted		17.54	24.62
Weighted Average Exercise Price, Exercised	13.76	10.70	14.12
Weighted Average Exercise Price, Canceled / forfeited	18.46	22.80	25.41
Ending Balance, Weighted Average Exercise Price, Outstanding	17.87	17.64	17.82
Weighted Average Exercise Price, Exercisable at December 31,	$ 17.56	16.19	14.48
Weighted average grant date fair value per share		$ 4.07	$ 7.46
Aggregate grant date fair value		$ 3,013	$ 3,134
Black-Scholes assumptions
Dividend yield		2.85%	1.94%
Risk free rate of return		1.34%	1.64%
Volatility		31.97%	36.38%
Expected life		6 years	6 years
Compensation expense	$ 1,371	$ 2,735	$ 2,645

Share based compensation arrangement by share based payment award options grant date fair value.

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- Definition

Share based compensation arrangement by share Based payment award options grant date fair value per share.

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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-Publisher FASB
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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Weighted average expected volatility rate of share-based compensation awards.

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- Definition

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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- Definition

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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- Definition

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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- Definition

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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- Definition

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

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

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- Definition

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

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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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- Definition

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

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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