Securities - Available for Sale Securities (Unrealized Loss Position) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	$ 342
Gross unrealized losses less than 12 months	342
Gross unrealized losses 12 months or longer
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses	1	1
Gross unrealized losses less than 12 months	1	1
Gross unrealized losses 12 months or longer	$ 1	$ 1	$ 1