Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) (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
Numerator (basic and diluted):
Net income	$ 146,120	$ 133,772	$ 293,612	$ 236,246
Denominator:
Denominator for basic earnings per share - weighted-average shares	124,870	137,399	125,920	138,982
Effect of stock options	2,318	2,317	2,341	2,307
Denominator for diluted earnings per share - weighted-average shares	127,188	139,716	128,261	141,289
Earnings per share - basic	$ 1.17	$ 0.97	$ 2.33	$ 1.70
Earnings per share - assuming dilution	$ 1.15	$ 0.96	$ 2.29	$ 1.67

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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-Number 210

-Section 03

-Paragraph 20

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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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-Name Regulation S-X (SX)

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-Publisher FASB

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-Glossary Other Comprehensive Income

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

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-Number 210

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

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-Name Accounting Standards Codification

-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)

-Number 51

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-Subparagraph a

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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

-Name Accounting Standards Codification

-Glossary Weighted-Average Number of Common Shares Outstanding

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

+ Details