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Quarterly Results Of Operations (Schedule Of Quarterly Results Of Operations) (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
Quarterly Results Of Operations [Abstract]                      
Operating revenues $ 507,937 $ 509,587 $ 515,897 $ 469,429 $ 463,214 $ 463,262 $ 463,469 $ 425,075 $ 2,002,850 $ 1,815,020 $ 1,666,470
Operating income 49,399 50,066 46,767 27,442 40,627 40,145 35,546 18,264 173,674 134,582 96,691
Net income $ 29,368 $ 29,578 $ 27,518 $ 16,293 $ 24,115 $ 24,158 $ 20,930 $ 10,836 $ 102,757 $ 80,039 $ 56,584
Basic earnings per share $ 0.40 $ 0.41 $ 0.38 $ 0.22 $ 0.33 $ 0.33 $ 0.29 $ 0.15 $ 1.41 $ 1.11 $ 0.79
Diluted earnings per share $ 0.40 $ 0.40 $ 0.38 $ 0.22 $ 0.33 $ 0.33 $ 0.29 $ 0.15 $ 1.40 $ 1.10 $ 0.79