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Employee Benefit Plans - Summary of Restricted Shares Issued, Vested and Forfeited (Detail)	12 Months Ended
	Dec. 31, 2018 Employee shares	Dec. 31, 2017 Employee shares	Dec. 31, 2016 Employee shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of shares of restricted stock, Shares | shares	33,000	32,000	32,300
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares vested | shares	28,790	24,230	20,050
Restricted shares awarded due to retirement | shares		640	2,394
Restricted shares forfeited | shares	2,620	1,080	1,546
Restricted shares issued | Employee	80	74	74
Restricted shares vested | Employee	56	52	45
Restricted shares awarded due to retirement | Employee		1	4
Restricted shares forfeited | Employee	5	2	2

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Number of awards granted to one employee on retirement.

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Share based compensation arrangement by share based payment award equity instruments other than options vested excludes shares awarded due to retirement.

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- Definition

Share Based Compensation Arrangement by share based payment award number of employee share awarded due to retirement.

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Share based compensation arrangement by share based payment award number of employee share issued.

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Share based compensation arrangement by share based payment award number of employee shares forfeited.

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Share based compensation arrangement by share based payment award number of employees.

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited 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)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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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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- Definition

Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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