Share-Based Compensation - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of share-based compensation plans	2
Closing sale price per share	42.16us-gaap_SaleOfStockPricePerShare	45.89us-gaap_SaleOfStockPricePerShare	30.99us-gaap_SaleOfStockPricePerShare
Stock options granted	5,538	585,189
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Payout based on economic profit performance against target economic profit goals	0.00%	0.00%	0.00%
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Payout based on economic profit performance against target economic profit goals	200.00%	200.00%	150.00%
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period, in years	2 years 3 months	3 years
Term of stock options, in years	10 years
Unrecognized compensation cost	2.5
Share-based compensation, weighed -average period for recognition	1 year 3 months
Stock options granted	5,538	585,189	0
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period, in years	3 years
Restricted stock and restricted stock units granted	253,042	181,916	461,512
Diluted earnings per share required to determine specific performance measure of stock awards	0.50
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period, in years	3 years
Unrecognized compensation cost	12.9
Share-based compensation, weighed -average period for recognition	2 years 1 month 6 days
Restricted stock and restricted stock units granted	176,717	166,348	245,737
Non Employee Directors [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense, directors fees	0.1	0.1	0.2
Accumulated number of stock units outstanding	43,082	56,737	62,421
Restricted stock and restricted stock units granted	20,664	23,898	41,051
Employees [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted stock and restricted stock units granted	271,594	194,517	426,064
Executive Officer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted stock and restricted stock units granted	137,501	129,849	240,134
Terms of granting restricted stock and restricted stock units	(1) the officer is still employed as of the anniversary date of the grant; and (2) the Company’s cumulative diluted earnings per share for the four calendar quarters immediately preceding the vesting date exceed $0.50 per diluted share as defined in the officers’ restricted stock agreement.

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Noninterest expense related to directors' fees which are fees paid by an Entity to its directors. Directors' fees may be paid in addition to salary and other benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.14)

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- Definition

Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

+ References

No definition available.

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- 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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&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://www.xbrl.org/2003/role/presentationRef

-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=6415400&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://www.xbrl.org/2003/role/presentationRef

-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=6415400&loc=d3e5070-113901

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- Definition

Gross number of share options (or share units) granted during the period.

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- Definition

Description of pertinent provisions of equity-based compensation awards that have actual or potential impact upon the company's financial statements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 50

-Paragraph 2

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- Definition

Number of share based compensation plans.

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- Definition

Payout percentage.

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- Definition

Amount of diluted earnings per share required to be attained over the previous four quarters immediately preceding the vesting date for use in determining achievement of specific performance measure of equity-based compensation awards.

+ References

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- Definition

The time period from the grant date until the equity-based award expires as specified in the award agreement.

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