Available-For-Sale Securities (Narrative) (Detail) $ in Billions	Sep. 30, 2015 USD ($) securities	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)
Available-for-sale Securities [Abstract]
Pledged Securities, carrying value	$ 1.3	$ 1.1	$ 1.1
Number of securities by a single non-goverment sponsored issuer exceeding 10% of shareholders' equity | securities	0