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Restricted Stock, Performance Share and Cash Awards - Schedule of Restricted Stock Awards Vesting Periods (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Vesting Period [Line Items]
Shares granted in the period	439,100	476,350	479,167
Vesting Period One Year [Member]
Vesting Period [Line Items]
Shares granted in the period	18,900	21,600	27,417
Vesting period, years	1 year
Vesting Period Two Years [Member]
Vesting Period [Line Items]
Shares granted in the period	12,700	12,750
Vesting period, years	2 years
Vesting Period Five Years [Member]
Vesting Period [Line Items]
Shares granted in the period	407,500	442,000	451,750
Vesting period, years	5 years

X

- Definition

Vesting Period [Line Items]

+ References

No definition available.

+ Details

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X

- 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

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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

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