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Benefit Plans (Summary Of Value Of Non-Vested Share Awards Granted And Vested) (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Weighted-average grant-date fair value of non-vested share awards granted (per share) $ 23.95 $ 51.22 $ 59.79
Total fair value of non-vested share awards vested during the period (in millions) $ 18.0 $ 46.2 $ 49.6