Consolidated Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Held-to-maturity securities, fair value	$ 11,565,000,000	$ 10,007,000,000
Unearned income on loans	736,000,000	826,000,000
Aggregate amount of certificates of deposit, denominations	$ 100,000	$ 100,000
Common shares, par value (in dollars per share)	$ 1	$ 1
Common shares, shares authorized (in shares)	1,400,000,000	1,400,000,000
Common shares, shares issued (in shares)	1,256,702,081	1,256,702,081
Treasury stock (in shares)	187,617,832	177,388,429
Loans held for sale	$ 0	$ 0
Residential Mortgage [Member]
Loans held for sale	$ 71,000,000	$ 62,000,000