Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Fair value of securities held to maturity	$ 10,207	$ 8,412
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	136,330,831	136,330,831
Common stock, share outstanding	136,330,831	136,330,831