Earnings per Share (Schedule of Net Income Attributable to Common Stockholders) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share [Abstract]
Net income	$ 86,964	$ 67,930	$ 290,278	$ 232,523
Less: Net income attributable to participating securities	(452)	(598)	(1,682)	(2,087)
Net income attributable to common shareholders	$ 86,512	$ 67,332	$ 288,596	$ 230,436