Stockholders' equity - Components of accumulated other comprehensive income (loss) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	$ 935
Balance at the end of period	1,246
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(120)
Other comprehensive (loss) income	6
Balance at the end of period	(114)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	1
Other comprehensive (loss) income	(1)
Unrecognized Loss on Interest Rate Swaps
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(20)
Other comprehensive (loss) income	6
Balance at the end of period	(14)
Pension Adjustments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(101)
Other comprehensive (loss) income	1
Balance at the end of period	$ (100)