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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accounting Policies [Abstract]
Net income for common stock	$ 165	$ 190	$ 584	$ 565
Weighted average common shares outstanding – basic (shares)	345.4	334.1	344.0	333.8
Add: Incremental shares attributable to effect of potentially dilutive securities (shares)	0.8	0.9	0.8	0.9
Adjusted weighted average common shares outstanding – diluted (shares)	346.2	335.0	344.8	334.7
Net income per common share - basic (dollars per share)	$ 0.48	$ 0.57	$ 1.70	$ 1.69
Net income per common share - diluted (dollars per share)	$ 0.48	$ 0.57	$ 1.70	$ 1.69