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Summarized Quarterly Financial Data (Schedule Of Quarterly Financial Information) (Details) (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
Summarized Quarterly Financial Data [Abstract]                      
Revenues $ 155,824 $ 135,865 $ 163,789 $ 171,956 $ 171,951 $ 147,597 $ 159,283 $ 157,901 $ 627,434 $ 636,732 $ 511,961
Income from operations 45,369 24,405 50,145 59,225 58,905 38,249 58,691 59,926 179,143 215,771 172,354
Net income $ 26,671 $ 13,935 $ 29,647 $ 35,791 $ 35,883 $ 23,341 $ 35,657 $ 36,379 $ 106,044 $ 131,260 $ 105,078
Basic $ 2.31 $ 1.20 $ 2.54 $ 2.81 $ 2.74 $ 1.73 $ 2.63 $ 2.68 $ 8.91 $ 9.78 $ 7.67
Diluted $ 2.30 $ 1.20 $ 2.53 $ 2.80 $ 2.73 $ 1.72 $ 2.60 $ 2.65 $ 8.88 $ 9.70 $ 7.60