Common and Preferred Shares and Warrants (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	904.0	969.0	1,023.0
Common shares repurchased	(50.0)	(70.0)	(58.8)
Other, primarily stock option exercises and RSAs granted	5.0	5.0	5.0
Shares issued and outstanding as of December 31	859.0	904.0	969.0
Stockholders' Equity Note (Textuals) [Abstract]
Shares reserved for issuance under employee stock and employee benefit plans	29.0