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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 215	$ 211	$ 113	$ (65)	$ 269	$ 292	$ 298	$ 339	$ 474	$ 1,198	$ 1,235
Less income allocated to participating securities	1	0	1	0	2	2	1	2	2	7	8
Preferred stock dividends									13	0	0
Net income attributable to common shares	$ 209	$ 203	$ 112	$ (65)	$ 267	$ 290	$ 297	$ 337	$ 459	$ 1,191	$ 1,227
Basic average common shares									139.0	150.0	168.0
Basic net income per common share	$ 1.50	$ 1.45	$ 0.81	$ (0.46)	$ 1.87	$ 1.98	$ 1.95	$ 2.14	$ 3.29	$ 7.95	$ 7.31
Basic average common shares									139.0	150.0	168.0
Net effect of the assumed exercise of stock options									1.0	1.0	2.0
Net effect of the assumed exercise of warrants									0.0	0.0	1.0
Diluted average common shares									140.0	151.0	171.0
Diluted net income per common share	$ 1.49	$ 1.44	$ 0.80	$ (0.46)	$ 1.85	$ 1.96	$ 1.94	$ 2.11	$ 3.27	$ 7.87	$ 7.20