Share-Based Payments - Components of Share-Based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	$ 15	$ 14	$ 44	$ 46
Stock options / stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	2	2	7	7
RSUs / DSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	8	7	25	24
PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	$ 5	$ 5	$ 12	$ 15