Summary of Non-Vested Shares (Detail) - $ / shares	3 Months Ended	9 Months Ended
Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Vested							(19,313)
Common Stock Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Beginning balance	628,231	632,660	391,753	403,096	559,587	324,756	391,753	324,756
Number of Shares, Granted	0	22,308	254,972	1,000	38,502	251,475
Number of Shares, Vested	(17,591)	(20,313)	(8,800)	(1,065)	(188,400)	(10,100)		(24,312)
Number of Shares, Forfeited	(13,659)	(6,424)	(5,265)	(5,209)	(6,593)	(6,544)
Number of Shares, Ending Balance	596,981	628,231	632,660	397,822	403,096	559,587	596,981	397,822
Weighted Average Grant-Date Fair Value, Beginning balance	$ 17.40	$ 17.41	$ 15.61	$ 15.61	$ 15.38	$ 14.75	$ 15.61	$ 14.75
Weighted Average Grant-Date Fair Value, Granted	0	23.66	19.97	19.90	17.08	16.10
Weighted Average Grant-Date Fair Value, Vested	13.51	22.82	12.07	12.88	15.22	12.95
Weighted Average Grant-Date Fair Value, Forfeited	17.35	17.25	16.51	15.80	15.55	15.26
Weighted Average Grant-Date Fair Value, Ending balance	$ 17.51	$ 17.40	$ 17.41	$ 15.63	$ 15.61	$ 15.38	$ 17.51	$ 15.63

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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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).

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The weighted average fair value at grant date for nonvested equity-based awards issued 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).

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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.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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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