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Share-Based Payments - Summary Of Change in Number of BCE 2010 Outstanding (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Balance at beginning of period 147,359 143,694  
Balance at end of period 181,008 147,359 143,694
BCE2010 [Member]      
Disclosure of terms and conditions of share-based payment arrangement [line items]      
Balance at beginning of period 12,339 15,974 29,972
Granted during the period 0 0 0
Forfeited during the period 0 0 0
Exercised during the period 1,334 3,635 13,998
Expired during the period 0 0 0
Balance at end of period 11,005 12,339 15,974