Summary of Significant Accounting Policies (Computation of Basic and Diluted Earnings per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Basic net income per share
Net income	$ 22,460	$ 23,277
Less income allocated to participating securities	(30)	(35)
Net income allocated to common stockholders	22,430	23,242
Weighted average common shares outstanding - basic	45,124	47,861
Net Income per share - basic	$ 0.50	$ 0.49
Diluted net income per share
Net income	22,460	23,277
Less income allocated to participating securities	(29)	0
Net income allocated to common stockholders	$ 22,431	$ 23,277
Weighted average common shares outstanding - basic	45,124	47,861
Dilutive securities	1,080	1,143
Weighted average common shares outstanding - diluted	46,204	49,004
Net Income per share - diluted	$ 0.49	$ 0.48