Available-For-Sale Securities (Narrative) (Detail) (USD $) In Billions, unless otherwise specified	Mar. 31, 2013 securities bonds	Dec. 31, 2012	Mar. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Number of bonds in a continuous unrealized loss position, twelve months or longer	7
Number of trust preferred securities in a continuous unrealized loss position, twelve months or longer	3
Securities, carrying value	$ 1.1	$ 1.1	$ 1.1
Number of securities by a single non-goverment sponsored issuer exceeding 10% of shareholders' equity	0