Net Income Per Share (Calculation of Net Income per Share) (Details) (USD $) In Thousands, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net income	$ 36,425	$ 12,181	$ 63,130	$ 20,656
Net income available to common stockholders-basic	35,888	11,660	62,191	19,286
Basic weighted average shares	141,286	143,100	141,966	143,667
Basic net income per share	$ 0.25	$ 0.08	$ 0.44	$ 0.13
Net income available to common stockholders-diluted	35,890	11,660	62,194	19,286
Dilutive effect of potential common shares	1,299	1,231	1,391	1,332
Diluted weighted average shares	142,585	144,331	143,357	144,999
Diluted net income per share	$ 0.25	$ 0.08	$ 0.43	$ 0.13
Basic [Member]
Income allocated to participating securities	537	521	939	1,370
Diluted [Member]
Income allocated to participating securities	$ 535	$ 521	$ 936	$ 1,370