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Summarized Quarterly Financial Data (Unaudited) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Schedule of quarterly financial information                      
Revenues $ 141,933 $ 124,260 $ 146,254 $ 149,532 $ 155,824 $ 135,865 $ 163,789 $ 171,956 $ 561,979 $ 627,434 $ 636,732
Income from operations 28,725 7,836 36,168 40,858 45,369 24,405 50,145 59,225 113,587 179,143 215,771
Net income $ 16,627 $ 4,103 $ 21,212 $ 23,989 $ 26,671 $ 13,935 $ 29,647 $ 35,791 $ 65,930 $ 106,044 $ 131,260
Net income per share:                      
Basic $ 1.47 $ 0.36 $ 1.86 $ 2.10 $ 2.31 $ 1.20 $ 2.54 $ 2.81 $ 5.79 $ 8.91 $ 9.78
Diluted $ 1.47 $ 0.36 $ 1.85 $ 2.09 $ 2.30 $ 1.20 $ 2.53 $ 2.80 $ 5.76 $ 8.88 $ 9.70