Stockholders' equity - Components of accumulated other comprehensive income (loss) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	$ 643
Balance at the end of period	579
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	82
Other comprehensive (loss) income	(5)
Tax expense	(1)
Balance at the end of period	76
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	162
Other comprehensive (loss) income	(9)
Balance at the end of period	153
Net (Loss) Income Hedge Investments in Foreign Operations
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(51)
Other comprehensive (loss) income	5
Balance at the end of period	(46)
Unrecognized (Loss) Gain on Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(4)
Other comprehensive (loss) income	(4)
Balance at the end of period	(8)
Pension Adjustments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(25)
Other comprehensive (loss) income	3
Tax expense	(1)
Balance at the end of period	$ (23)