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, 2019	Mar. 31, 2018
Accounting Policies [Abstract]
Net income for common stock	$ 424	$ 428
Weighted average common shares outstanding – basic (shares)	322.5	310.4
Add: Incremental shares attributable to effect of potentially dilutive securities (shares)	0.9	1.2
Adjusted weighted average common shares outstanding – diluted (shares)	323.4	311.6
Net income per common share - basic (dollars per share)	$ 1.31	$ 1.38
Net income per common share - diluted (dollars per share)	$ 1.31	$ 1.37