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Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail) - shares | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Vesting Period [Line Items] |  |  |
Shares granted in the period | 476,350 | 479,167 | 394,975
Vesting Period One Year [Member] |  |  |
Vesting Period [Line Items] |  |  |
Shares granted in the period | 21,600 | 27,417 | 22,175
Vesting period, years | 1 year |  |
Vesting Period Four Years [Member] |  |  |
Vesting Period [Line Items] |  |  |
Shares granted in the period |  |  | 9,200
Vesting period, years | 4 years |  |
Vesting Period Five Years [Member] |  |  |
Vesting Period [Line Items] |  |  |
Shares granted in the period | 454,750 | 451,750 | 363,600
Vesting period, years | 5 years |  |

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- Definition Vesting Period [Line Items] + References No definition available. + 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://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 + 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://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 + Details
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- Details
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- Details
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- Details
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