Net Income Per Common Share (Basic And Diluted Net Income Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 226	$ 149	$ 631	$ 313
Income allocated to participating securities	2	1	5	3
Net income attributable to common shares	$ 224	$ 148	$ 626	$ 310
Basic average common shares	174	171	175	173
Basic net income per common share	$ 1.29	$ 0.87	$ 3.58	$ 1.80
Basic average common shares	174	171	175	173
Net effect of the assumed exercise of stock options	2	2	3	1
Net effect of the assumed exercise of warrants	1	3	1	2
Diluted average common shares	177	176	179	176
Diluted net income per common share	$ 1.26	$ 0.84	$ 3.50	$ 1.76