Summarizes Restricted Stock Unit Activity (Detail) - shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding at beginning of period, Shares	282,100	210,481	179,026
Granted, Shares	21,681	237,542	141,784
Vested, Shares	(130,684)	(165,923)	(100,330)
Forfeited, Shares	(36,531)		(9,999)
Outstanding at end of period, Shares	136,566	282,100	210,481
RSU
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	215,969	0
Outstanding at end of period, Shares	215,969
RSU \| Time Vesting Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	86,392
Outstanding at end of period, Shares	86,392
RSU \| ROIC Target Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	64,785
Outstanding at end of period, Shares	64,785
RSU \| TSR Target Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	64,792
Outstanding at end of period, Shares	64,792

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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)(1)(iv)(3)
-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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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet 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 (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ 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)(2)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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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.

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