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Summary of Performance Stock Units Activity (Detail) - Performance Stock Units PSU
shares in Thousands
12 Months Ended
Dec. 31, 2017
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Unvested at beginning of year 195
Granted 87
Vested (59)
Forfeited (48)
Unvested the end of the year 175