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Share-Based Payments - SShare-Based Payments - Summary Of Change in Number of BSA 2010 Outstanding (Detail)
 12 Months Ended
Jun. 24, 2011
Dec. 31, 2017
Dec. 15, 2017
Dec. 05, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Granted during the period     8,300 625,200    
BSA2010 [Member]            
Disclosure of terms and conditions of share-based payment arrangement [line items]            
Balance at beginning of period   610     1,044 8,334
Granted during the period 2,000 0     0 0
Forfeited during the period   0     0 0
Exercised during the period   110     434 7,290
Expired during the period   0     0 0
Balance at end of period   500     610 1,044