Investment Securities (Credit Losses on Non-Agency Residential Mortgage-Backed Securities) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Unrealized Loss And Fair Value Available For Sale Securities [Line Items]
Balance	$ 1,566	$ 4,582	$ 4,602	$ 1,737
Amount related to credit losses for which an other-than-temporary impairment was not previously recognized	0	215	0	2,791
Additional amount related to credit losses for which an other-than-temporary impairment was previously recognized	0	0	144	269
Sale of securities for which other-than-temporary impairment was previously recognized	0	0	3,180	0
Balance	$ 1,566	$ 4,797	$ 1,566	$ 4,797