Share-Based Compensation - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Capitalized share-based compensation cost	$ 0	$ 0
Number of stock options granted	0	93,955
Weighted-average exercise price for stock options granted		$ 33.87
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation options awarded, vesting period	Stock options awarded to employees under the Company's 2008 Equity Compensation Plan ("2008 Plan") generally vest annually over a three-year period
Stock options term in years	10 years
Unrecognized compensation cost	2.5
Weighted-average period for which cost is expected to be recognized, in years	1 year 10 months 24 days
Weighted-average exercise price for outstanding stock options	$ 33.69
Weighted-average remaining contractual term for outstanding stock options	5 years 7 months 6 days
Service Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	8.1
Weighted-average period for which cost is expected to be recognized, in years	3 years 6 months
Share-based compensation awarded, number of installments	4
Performance Award Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	1.1
Weighted-average period for which cost is expected to be recognized, in years	2 years 3 months 18 days
Share-based compensation awarded, number of installments	4
DSU Service Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	0
Share-based compensation awarded, vesting period	4 years
Dsu Performance Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awarded, number of installments	4
Board Dsus Granted To Nonemployee Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost	$ 0.5
Board Dsus Granted To Nonemployee Directors \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average period for which cost is expected to be recognized, in years	1 year
One Executive
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awarded, number of installments	12
Other Executives
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awarded, number of installments	16
Board DSUs And Dividend Equivalents Granted To Board
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation awarded, number of installments	3
Share-based compensation awarded, vesting period	3 years

Share Based Compensation Arrangement By Share Based Payment Award Award Vesting Description

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- Definition

Share-based Compensation Arrangement By Share-based Payment Award, Award Vesting, Number Of Installments

+ References

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award, Options, Vesting Period

+ References

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- Definition

Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

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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

As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

+ References

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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

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.

+ References

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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.

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- Definition

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

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-Paragraph 2

-Subparagraph (c)(1)(iv)(1)

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-Paragraph A240

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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

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

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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

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

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- Definition

Weighted average price at which grantees can acquire the shares reserved for issuance on stock options awarded.

+ References

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