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								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 116	$ 136	$ 135	$ 134	$ 149	$ 154	$ 151	$ 139	$ 521	$ 593	$ 541
Less income allocated to participating securities	1	2	1	2	1	2	2	2	6	7	8
Net income attributable to common shares	$ 115	$ 134	$ 134	$ 132	$ 148	$ 152	$ 149	$ 137	$ 515	$ 586	$ 533
Basic average common shares									176	179	183
Basic net income per common share	$ 0.65	$ 0.76	$ 0.76	$ 0.75	$ 0.83	$ 0.85	$ 0.83	$ 0.76	$ 2.93	$ 3.28	$ 2.92
Basic average common shares									176	179	183
Net effect of the assumed exercise of stock options									2	2	1
Net effect of the assumed exercise of warrants									3	4	3
Diluted average common shares									181	185	187
Diluted net income per common share	$ 0.64	$ 0.74	$ 0.73	$ 0.73	$ 0.80	$ 0.82	$ 0.80	$ 0.73	$ 2.84	$ 3.16	$ 2.85