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Contract costs (Tables)
 12 Months Ended
Dec. 31, 2018
Revenue From Contract With Customer1 [Abstract]  
Contract costs
The table below provides a reconciliation of the contract costs balance.
FOR THE YEAR ENDED DECEMBER 31
2018

2017

Opening balance, January 1
636

618

Incremental costs of obtaining a contract and contract fulfillment costs
567

526

Amortization included in operating costs
(477
)
(508
)
Impairment charges included in operating costs
(19
)
—

Ending balance, December 31
707

636