Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Trading account assets pledged as collateral	$ 2,878	$ 1,001
Securities available-for-sale pledged as collateral	101	163
Fair value of securities held to maturity	$ 10,729	$ 9,799
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (dollars per share)	$ 1	$ 1
Common stock, share authorized (in shares)	300,000,000	300,000,000
Common stock, share issued (in shares)	147,589,713
Common stock, share outstanding (in shares)	147,589,713	147,589,713