Share-Based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based compensation expense
Pre-tax compensation expense	$ 7	$ 7	$ 20	$ 21
Income tax benefit	(4)	(3)	(8)	(8)
Total share-based compensation expense, net of income taxes	3	4	12	13
Employee Stock Option
Share-based compensation expense
Pre-tax compensation expense	2	2	6	7
Income tax benefit	(1)	(1)	(2)	(3)
Total share-based compensation expense, net of income taxes	1	1	4	4
Restricted Stock Units (RSUs)
Share-based compensation expense
Pre-tax compensation expense	4	3	10	9
Income tax benefit	(2)	(1)	(4)	(3)
Total share-based compensation expense, net of income taxes	2	2	6	6
Performance shares and other share-based awards
Share-based compensation expense
Pre-tax compensation expense	1	2	4	5
Income tax benefit	$ (1)	(1)	(2)	(2)
Total share-based compensation expense, net of income taxes		$ 1	$ 2	$ 3