Common and Preferred Shares and Warrants (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Authorized shares and a reconciliation of common shares issued and outstanding
Common shares, authorized	3,600.0	3,600.0	3,600.0
Shares issued and outstanding at beginning of year	859.0	904.0	969.0
Common shares repurchased	(15.0)	(50.0)	(70.0)
Other, primarily stock option exercises and RSAs granted	3.0	5.0	5.0
Shares issued and outstanding as of December 31	847.0	859.0	904.0
Stockholders' Equity Note (Textuals) [Abstract]
Shares reserved for issuance under employee stock and employee benefit plans	24.0