Share-Based Payments - Components of Share-Based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	$ 16	$ 16	$ 29	$ 32
Stock options / stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	3	2	5	5
RSUs / DSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	9	9	17	17
PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense—direct	$ 4	$ 5	$ 7	$ 10