Annual report pursuant to Section 13 and 15(d)

Quarterly Information (Details Textual)

v2.4.0.6
Quarterly Information (Details Textual) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Quarterly Information Textual Abstract                      
Net Income $ 105,400 $ 108,000 $ 111,200 $ 88,000 $ 86,300 $ 78,900 $ 82,200 $ 51,800 $ 412,578 $ 299,215 $ 203,346
Earnings per share basic $ 0.91 $ 0.92 $ 0.93 $ 0.74 $ 0.73 $ 0.67 $ 0.69 $ 0.43 $ 3.51 $ 2.51 $ 1.66
Earning per share diluted $ 0.86 $ 0.87 $ 0.88 $ 0.70 $ 0.69 $ 0.63 $ 0.65 $ 0.41 $ 3.30 $ 2.37 $ 1.61
As Reported [Member]
                      
Quarterly Information Textual Abstract                      
Net Income       $ 87,600 $ 81,000         $ 290,533  
Earnings per share basic       $ 0.74 $ 0.69         $ 2.44  
Earning per share diluted       $ 0.71 $ 0.65         $ 2.33