Accumulated other comprehensive income (loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	$ 1,832
Cumulative-effect of IFRS to U.S. GAAP conversion			$ (97)
Balance at the end of period	643	$ 1,832
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	113	123	169
Cumulative-effect of IFRS to U.S. GAAP conversion			(33)
Other comprehensive (loss) income	(33)	(4)	(15)
Tax expense (recovery)	2	(6)	2
Balance at the end of period	82	113	123
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	158	151	154
Cumulative-effect of IFRS to U.S. GAAP conversion			(1)
Other comprehensive (loss) income	4	7	(2)
Balance at the end of period	162	158	151
Net (Loss) Income Hedge Investments in Foreign Operations
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(28)	(28)	(33)
Other comprehensive (loss) income	(23)		5
Balance at the end of period	(51)	(28)	(28)
Unrecognized (Loss) Gain on Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	7	10	18
Other comprehensive (loss) income	(10)	(3)	(9)
Tax expense (recovery)	3		1
Balance at the end of period		7	10
Pension Liability Adjustments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(24)	(10)	30
Cumulative-effect of IFRS to U.S. GAAP conversion			(32)
Other comprehensive (loss) income	(4)	(8)	(9)
Tax expense (recovery)	(1)	(6)	1
Balance at the end of period	$ (29)	$ (24)	$ (10)