Acquisitions of Real Estate Property (Details 8) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
The effect of material acquisitions on net income and earnings per share
Revenues	$ 2,256,319	$ 2,178,897
Income from continuing operations attributable to common stockholders	$ 583,446	$ 321,637
Basic:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 2.03	$ 1.14
Diluted:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 2.02	$ 1.14
Weighted average shares used in computing earnings per common share:
Basic (in shares)	286,856	281,333
Diluted (in shares)	289,193	282,382

The pro forma income from continuing operations before changes in accounting and extraordinary items, net of tax effect per basic share of common stock for a period as if the business combination or combinations had been completed at the beginning of a period.

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