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, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Net income	$ 1,343	$ 1,382	$ 1,525
Weighted average common shares outstanding – basic (in shares)	328.5	311.7	307.1
Add: Incremental shares attributable to effect of potentially dilutive securities (in shares)	1.0	1.2	1.7
Adjusted weighted average common shares outstanding – diluted (in shares)	329.5	312.9	308.8
Net income per common share — basic (in dollars per share)	$ 4.09	$ 4.43	$ 4.97
Net income per common share — diluted (in dollars per share)	$ 4.08	$ 4.42	$ 4.94