Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Statement Of Financial Position [Abstract]
Trading account assets pledged as collateral	$ 4	$ 1
Fair value of securities held to maturity	$ 891	$ 1,135
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 1	$ 1
Common stock, share authorized	300,000,000	300,000,000
Common stock, share issued	136,330,830	136,330,830
Common stock, share outstanding	136,330,830	136,330,830