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Stock-Based Compensation - Additional Information (Detail) - shares | 9 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017
Restricted Stock Units (RSU) [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
The vesting graded scale | The RSUs vest on the passage of time, generally on a graded scale over a three, four, and five-year period, or in certain instances the RSUs were issued with five year cliff vesting. |
Performance Share Awards (PSAs) [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Cumulative performance targets | 2 years |
Additional service period | 1 year |
Grants under performance share awards | 184,121 | 163,274
Stock Only Stock Appreciation Rights (SOSARs) [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Vesting period of stock | 3 years |
Vesting term | 10 years |
Share-based payment award, shares issued in period | 0 | 0

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- Definition Share based compensation arrangement by share based payment award equity instruments other than options additional service period. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award equity instruments other than options cumulative performance target period. + References No definition available. + Details
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- Definition Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition 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://fasb.org/us-gaap/role/ref/legacyRef -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=109197908&loc=d3e5070-113901 + Details
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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. + References No definition available. + Details
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- Definition Number of share instruments newly issued under a share-based compensation plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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