Shared-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options granted	57,800
Weighted average grant date fair value of the options granted \| $ / shares	$ 10.60
Total grant-date fair value of restricted stock \| $	$ 1.8
Dividends accrued for performance share units \| $	$ 0.1
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock units issued	86,516
Fair value assumptions, exercise price \| $ / shares	$ 24.55
TSR Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock units issued	55,000
Percentage of shares issuable based on threshold performance	50.00%
Percentage of shares issuable based on target performance	100.00%
Percentage of shares issuable based on maximum performance	150.00%
Threshold performance, percentage	35.00%
ROIC Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock units issued	55,000
Percentage of shares issuable based on threshold performance	35.00%
Percentage of shares issuable based on target performance	100.00%
Percentage of shares issuable based on maximum performance	150.00%
Threshold performance, percentage	15.50%
Officers and Key Employees \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted vesting period	3 years
Non-executive Directors \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards granted vesting period	1 year

Dividends Accrued For Performance Share Units.

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Share based compensation arrangement by share based payment award based on threshold performance achievement percentage.

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Share based compensation arrangement by share based payment award percentage of shares issuable based on threshold performance.

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Share based compensation arrangement by share-based payment award percentage of shares issuable based on maximum performance.

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Share based compensation arrangement by share-based payment award, percentage of shares Issuable based on target performance.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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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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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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