Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Trading account assets pledged as collateral	$ 1,122	$ 426
Securities available-for-sale pledged as collateral	148	205
Fair value of securities held to maturity	$ 10,316	$ 10,207
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 (in dollars per share)	$ 1	$ 1
Common stock, share authorized (in shares)	300,000,000	300,000,000
Common stock, share issued (in shares)	144,322,280	144,322,280
Common stock, share outstanding (in shares)	144,322,280	144,322,280