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Shared-based Compensation (Summary of Restricted Stock Activity) (Detail) - Restricted Stock [Member] - $ / shares
shares in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Shares    
Unvested at beginning of period 129 215
Granted 16 44
Vested (69) (55)
Forfeited   (6)
Unvested at end of period 76 198
Weighted-Average Grant Date Fair Value    
Unvested at beginning of period $ 32.15 $ 27.69
Granted 66.68 44.73
Vested 28.35 25.94
Forfeited   32.80
Unvested at end of period $ 42.99 $ 31.79