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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 269	$ 292	$ 298	$ 339	$ 310	$ 318	$ 281	$ 326	$ 1,198	$ 1,235	$ 743
Less income allocated to participating securities	2	2	1	2	2	2	2	2	7	8	5
Net income attributable to common shares	$ 267	$ 290	$ 297	$ 337	$ 308	$ 316	$ 279	$ 324	$ 1,191	$ 1,227	$ 738
Basic average common shares									150	168	174
Basic net income per common share	$ 1.87	$ 1.98	$ 1.95	$ 2.14	$ 1.91	$ 1.89	$ 1.62	$ 1.90	$ 7.95	$ 7.31	$ 4.23
Basic average common shares									150	168	174
Net effect of the assumed exercise of stock options									1	2	3
Net effect of the assumed exercise of warrants									0	1	1
Diluted average common shares									151	171	178
Diluted net income per common share	$ 1.85	$ 1.96	$ 1.94	$ 2.11	$ 1.88	$ 1.86	$ 1.59	$ 1.87	$ 7.87	$ 7.20	$ 4.14