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Share-Based Compensation (Summary of Restricted Stock Activity) (Details) - Restricted stock - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Number of Awards      
Unvested balances at beginning of period (in shares) 1,988,500 2,560,578 3,343,738
Granted (in shares) 550,544 737,864 724,200
Vested (in shares) (702,832) (1,102,823) (1,238,980)
Forfeited (in shares) (252,837) (207,119) (268,380)
Unvested balances at end of period (in shares) 1,583,375 1,988,500 2,560,578
Weighted-Average Grant Date Fair Value      
Unvested balances at beginning of period (in dollars per share) $ 57.34 $ 45.92 $ 35.36
Granted (in dollars per share) 81.66 67.48 62.91
Vested (in dollars per share) 48.64 38.56 27.91
Forfeited (in dollars per share) 63.86 52.29 43.29
Unvested balances at end of period (in dollars per share) $ 68.62 $ 57.34 $ 45.92