Computation of Basic and Diluted Income Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Earnings Per Share [Line Items]
Net income	$ 45,154	$ 39,368	$ 62,998	$ 76,785
Basic common shares outstanding	96,266	100,094	96,432	100,416
Stock options and unvested restricted stock	1,145	1,034	1,164	1,102
Diluted common shares outstanding	97,411	101,128	97,596	101,518
Basic income per common share	$ 0.47	$ 0.39	$ 0.65	$ 0.76
Diluted income per common share	$ 0.46	$ 0.39	$ 0.65	$ 0.76

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

+ References+ Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

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

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-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

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-Glossary Net Income

-URI http://asc.fasb.org/extlink&oid=6518256

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-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

-Number 51

-Paragraph A7

-Appendix A

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-Name Accounting Research Bulletin (ARB)

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+ Details

- Definition

The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

+ References+ Details

- Definition

Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

+ References

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Glossary Weighted-Average Number of Common Shares Outstanding

-URI http://asc.fasb.org/extlink&oid=6528421

+ Details