Common and Preferred Shares (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Common shares, authorized (in shares)	3,600	3,600	3,600
Authorized shares and a reconciliation of common shares issued and outstanding
Shares issued and outstanding at beginning of year (in shares)	810	847	859
Common shares repurchased (in shares)	(7)	(40)	(15)
Other, primarily stock option exercises and RSAs granted (in shares)	2	3	3
Shares issued and outstanding as of December 31 (in shares)	805	810	847
Stockholders' Equity Note (Textuals) [Abstract]
Shares reserved for issuance under employee stock and employee benefit plans	23