Changes in Deferred Stock Units Related to Annual Grants (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
SARs, Granted	244,506
Non-Management Director Equity-Based Compensation
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
SARs, Granted	18,790	30,540	32,923
SARs, Exercised	(18,790)	(30,540)	(32,923)
Weighted Average Grant Date Fair Value, Granted	$ 18.56	$ 12.30	$ 9.42
Weighted Average Grant Date Fair Value, Vested	$ 18.56	$ 12.30	$ 9.42
Non-Management Director Equity-Based Compensation \| Deferred Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
SARs, Outstanding, Beginning balance	29,250	41,250	52,500
SARs, Granted	22,500	29,250	41,250
SARs, Exercised	(29,250)	(41,250)	(52,500)
SARs, Outstanding, Ending balance	22,500	29,250	41,250
Weighted Average Fair Value, Outstanding, Beginning balance	$ 13.09	$ 8.40	$ 13.32
Weighted Average Grant Date Fair Value, Granted	$ 17.63	$ 13.09	$ 8.40
Weighted Average Grant Date Fair Value, Vested	$ 13.09	$ 8.40	$ 13.32
Weighted Average Fair Value, Outstanding, Ending balance	$ 17.63	$ 13.09	$ 8.40

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).

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 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).

+ References

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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.

+ 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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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.

+ 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)

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(2)

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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.

+ 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)(2)

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