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Shareholders' Equity and Equity Compensation (Time Based Restricted Stock Awards) (Detail) - Time Based Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Awards Outstanding      
Non-vested awards outstanding, beginning balance (in shares) 239,189 208,318 238,386
Awards granted (in shares) 80,535 118,660 75,160
Stock issued (in shares) (140,208) (60,326) (93,813)
Awards forfeited or expired (in shares) (6,185) (27,463) (11,415)
Non-vested awards outstanding, ending balance (in shares) 173,331 239,189 208,318
Weighted- Average Grant Date Fair Value      
Weighted-Average Grant Date Fair Value, Outstanding at period beginning (in dollars per share) $ 57.71 $ 64.27 $ 53.80
Weighted-Average Grant Date Fair Value, Awards granted (in dollars per share) 83.17 51.70 77.15
Weighted-Average Grant Date Fair Value, Stock issued (in dollars per share) 58.18 64.03 48.35
Weighted-Average Grant Date Fair Value, Awards forfeited or expired (in dollars per share) 60.70 64.60 61.32
Weighted-Average Grant Date Fair Value, Outstanding at period end (in dollars per share) $ 69.10 $ 57.71 $ 64.27