Summary of Significant Accounting Policies and Other Matters - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Net income	$ 1,101	$ 1,343	$ 1,382	$ 1,525
Weighted average common shares outstanding – basic (in shares)	334.8	328.5	311.7
Add: Incremental shares attributable to effect of potentially dilutive securities (in shares)	0.9	1.0	1.2
Adjusted weighted average common shares outstanding – diluted (in shares)	335.7	329.5	312.9
Net income per common share — basic (in dollars per share)	$ 3.29	$ 4.09	$ 4.43
Net income per common share — diluted (in dollars per share)	$ 3.28	$ 4.08	$ 4.42