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
	Mar. 31, 2020	Mar. 31, 2019
Accounting Policies [Abstract]
Net income for common stock	$ 375	$ 424
Weighted average common shares outstanding – basic (shares)	333.6	322.5
Add: Incremental shares attributable to effect of potentially dilutive securities (shares)	1.0	0.9
Adjusted weighted average common shares outstanding – diluted (shares)	334.6	323.4
Net income per common share - basic (dollars per share)	$ 1.13	$ 1.31
Net income per common share - diluted (dollars per share)	$ 1.12	$ 1.31