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Employee Benefits - Stocks Options Outstanding by Grant Date (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Outstanding, shares (in Shares)	31,949	31,949	41,787	46,451	58,581
Outstanding, average exercise price	$ 25.03	$ 25.03	$ 27.27	$ 27.90	$ 28.38
Exercisable, shares (in Shares)	31,949	31,949	41,787	46,451
Exercisable, average exercise price	$ 25.03	$ 25.03	$ 27.27	$ 27.90
Granted December 10, 2006 [Member]
Exercise price	$ 40.24
Outstanding, shares (in Shares)	2,625
Outstanding, contractual average life	11 months 9 days
Outstanding, average exercise price	$ 40.24
Exercisable, shares (in Shares)	2,625
Exercisable, average exercise price	$ 40.24
Granted May 16, 2007 [Member]
Exercise price	$ 37.48
Outstanding, shares (in Shares)	1,337
Outstanding, contractual average life	1 year 4 months 13 days
Outstanding, average exercise price	$ 37.48
Exercisable, shares (in Shares)	1,337
Exercisable, average exercise price	$ 37.48
Granted December 10, 2007 [Member]
Exercise price	$ 37.00
Outstanding, shares (in Shares)	1,950
Outstanding, contractual average life	1 year 11 months 9 days
Outstanding, average exercise price	$ 37.00
Exercisable, shares (in Shares)	1,950
Exercisable, average exercise price	$ 37.00
Granted January 2, 2008 [Member]
Exercise price	$ 36.25
Outstanding, shares (in Shares)	1,337
Outstanding, contractual average life	2 years
Outstanding, average exercise price	$ 36.25
Exercisable, shares (in Shares)	1,337
Exercisable, average exercise price	$ 36.25
Granted November 10, 2008 [Member]
Exercise price	$ 23.00
Outstanding, shares (in Shares)	16,500
Outstanding, contractual average life	2 years 10 months 10 days
Outstanding, average exercise price	$ 23.00
Exercisable, shares (in Shares)	16,500
Exercisable, average exercise price	$ 23.00
Granted May 9, 2011 [Member]
Exercise price	$ 17.55
Outstanding, shares (in Shares)	8,200
Outstanding, contractual average life	5 years 4 months 6 days
Outstanding, average exercise price	$ 17.55
Exercisable, shares (in Shares)	8,200
Exercisable, average exercise price	$ 17.55

X

- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option 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)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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X

- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option 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)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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X

- Definition

Number of options outstanding, including both vested and non-vested options.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option 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)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

+ References

No definition available.

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- Definition

Weighted average remaining contractual term for option awards outstanding, 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 (e)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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