Summary of Significant Accounting Policies and Other Matters - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accounting Policies [Abstract]
Net income for common stock	$ 493	$ 473	$ 1,058	$ 1,048
Weighted average common shares outstanding – basic (shares)	334.5	332.2	334.1	327.3
Add: Incremental shares attributable to effect of potentially dilutive securities (shares)	0.9	1.0	0.9	1.0
Adjusted weighted average common shares outstanding – diluted (shares)	335.4	333.2	335.0	328.3
Net income per common share - basic (dollars per share)	$ 1.47	$ 1.42	$ 3.17	$ 3.20
Net income per common share - diluted (dollars per share)	$ 1.47	$ 1.42	$ 3.16	$ 3.19