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Common Stock (Details 4) (USD $)
1 Months Ended 12 Months Ended
Feb. 28, 2011
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Summary of information regarding outstanding restricted shares        
Restricted shares outstanding, Beginning balance   1,765,837 2,181,244 1,460,496
Restricted stock granted, shares   374,425 558,531 762,692
Restricted stock vested, shares (234,620) (496,642) (921,454) (7,322)
Restricted stock forfeited, shares   (37,573) (52,484) (34,622)
Restricted shares outstanding, Ending balance   1,606,047 1,765,837 2,181,244
Weighted-average grant date fair value of restricted stock, Ending balance   $ 44.21 $ 42.15 $ 40.51
Restricted stock granted, Weighted-average grant date fair value   $ 79.53 $ 43.99 $ 26.02
Restricted stock vested, Weighted-average grant date fair value   $ 64.22 $ 43.28 $ 36.05
Restricted stock forfeited, Weighted-average grant date fair value   $ 44.02 $ 35.11 $ 41.52
Weighted-average grant date fair value of restricted stock, Beginning balance   $ 42.15 $ 40.51 $ 47.34