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Shared-based Compensation (Summary of Restricted Stock Activity) (Detail) - Restricted Stock [Member] - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Shares    
Unvested at beginning of period 215 264
Granted 44 92
Vested (120) (102)
Forfeited (10) (39)
Unvested at end of period 129 215
Weighted-Average Grant Date Fair Value    
Unvested at beginning of period $ 27.69 $ 26.38
Granted 44.73 30.61
Vested 28.27 27.42
Forfeited 37.77 26.43
Unvested at end of period $ 32.15 $ 27.69