Stockholders' equity - Components of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period			$ 935
Reclassification to gain on disposal of discontinued operations				$ (64)
Balance at the end of period	$ 1,329
Accumulated other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(109)	$ (114)	(120)
Other comprehensive (loss) income	4	5	6
Balance at the end of period	(105)	(109)	(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	(10)	(14)	(20)
Other comprehensive (loss) income	3	4	6
Balance at the end of period	(7)	(10)	(14)
Pension Adjustments
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of period	(99)	(100)	(101)
Other comprehensive (loss) income	1	1	1
Balance at the end of period	$ (98)	$ (99)	$ (100)