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Earnings Per Share (Details) (USD $)
In Thousands, except 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
Earnings Per Share [Abstract]                      
Net income $ 33,698 $ 71,361 $ 52,259 $ 49,589 $ 49,412 $ 45,719 $ 39,856 $ 42,138 $ 206,907 $ 177,125 $ 162,283
Weighted-average number of shares of common stock outstanding 13,405,000 13,680,000 13,852,000 13,890,000 14,209,000 14,353,000 14,796,000 15,118,000 13,707,000 14,619,000 15,206,000
Basic earnings per share $ 2.51 $ 5.22 $ 3.77 $ 3.57 $ 3.48 $ 3.19 $ 2.69 $ 2.79 $ 15.10 $ 12.12 $ 10.67
Shares issuable upon exercise of stock options                 5,000 31,000 37,000
Total shares 13,405,000 13,680,000 13,856,000 13,906,000 14,235,000 14,383,000 14,828,000 15,154,000 13,712,000 14,650,000 15,243,000
Diluted earnings per share $ 2.51 $ 5.22 $ 3.77 $ 3.57 $ 3.47 $ 3.18 $ 2.69 $ 2.78 $ 15.09 $ 12.09 $ 10.65
Anti-dilutive options that were excluded from calculation of earnings per share                 0 0 0