Stock-Based Compensation - Summarizes Information Related to Performance Share Units (Detail) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Schedule Of Other Than Options (Line Items)
Non-vested shares outstanding, Beginning Balance	391,278	240,345
Granted	753,285	248,228
Vested	396,073	92,115
Forfeited	48,066	5,180
Non-vested shares outstanding, Ending Balance	700,424	391,278
Non-vested shares outstanding at December 31, Beginning Balance, Weighted Average Grant Date Fair Value	$ 25.40	$ 16.16
Granted, Weighted Average Grant Date Fair Value per Unit	$ 29.66	$ 30.99
Vested, Weighted Average Grant Date Fair Value per Unit	$ 26.04	$ 16.24
Forfeited, Weighted Average Grant Date Fair Value per Unit	$ 31.15	$ 27.40
Non-vested shares outstanding at December 31, Ending Balance, Weighted Average Grant Date Fair Value per Share	$ 29.22	$ 25.40
Performance Share Unit Awards [Member]
Schedule Of Other Than Options (Line Items)
Non-vested shares outstanding, Beginning Balance
Granted	155,220
Vested
Forfeited
Non-vested shares outstanding, Ending Balance	155,220
Granted, Weighted Average Grant Date Fair Value per Unit	$ 26.22
Vested, Weighted Average Grant Date Fair Value per Unit
Forfeited, Weighted Average Grant Date Fair Value per Unit
Non-vested shares outstanding at December 31, Ending Balance, Weighted Average Grant Date Fair Value per Share	$ 26.22

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

+ References

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- Definition

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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- 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

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

-Publisher FASB

-Name Accounting Standards Codification

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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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

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- Definition

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

-Publisher FASB

-Name Accounting Standards Codification

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-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

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

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- Definition

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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