Net Income Per Common Share (Calculations For Basic And Diluted Earnings Per 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
Numerator:
Net Income	$ 1,049	$ 484	$ 909	$ (1,226)	$ 626	$ 248	$ 428	$ 695	$ 1,216	$ 1,997	$ 287
Denominator:
Basic weighted-average common shares outstanding									161.2	163.2	163.2
Diluted weighted-average shares									162.4	165.1	164.5
Earnings per share:
Basic (in usd per share)	$ 6.52	$ 3.01	$ 5.65	$ (7.56)	$ 3.84	$ 1.51	$ 2.62	$ 4.27	$ 7.55	$ 12.24	$ 1.76
Diluted (in usd per share)	$ 6.47	$ 2.99	$ 5.63	$ (7.56)	$ 3.79	$ 1.49	$ 2.59	$ 4.22	$ 7.49	$ 12.10	$ 1.75
Number of anti-dilutive share-based awards									1.4	0.0	1.3
Stock Options
Denominator:
Effect of stock-based awards									0.7	1.2	0.8
Restricted Stock Units (RSUs)
Denominator:
Effect of stock-based awards									0.5	0.7	0.5