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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accounting Policies [Abstract]
Net income for common stock	$ 190	$ 152	$ 565	$ 576
Weighted average common shares outstanding – basic (shares)	334.1	328.3	333.8	325.2
Add: Incremental shares attributable to effect of potentially dilutive securities (shares)	0.9	0.9	0.9	0.9
Adjusted weighted average common shares outstanding – diluted (shares)	335.0	329.2	334.7	326.1
Net income per common share - basic (dollars per share)	$ 0.57	$ 0.46	$ 1.69	$ 1.77
Net income per common share - diluted (dollars per share)	$ 0.57	$ 0.46	$ 1.69	$ 1.77