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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Net income for common stock	$ 473	$ 435	$ 1,048	$ 1,051
Weighted average common shares outstanding – basic (shares)	332.2	311.1	327.3	310.8
Add: Incremental shares attributable to effect of potentially dilutive securities (shares)	1.0	1.2	1.0	1.1
Adjusted weighted average common shares outstanding – diluted (shares)	333.2	312.3	328.3	311.9
Net income per common share - basic (dollars per share)	$ 1.42	$ 1.40	$ 3.20	$ 3.38
Net income per common share - diluted (dollars per share)	$ 1.42	$ 1.39	$ 3.19	$ 3.37