Summary of Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Allowance for non-covered loans	$ 119,755		$ 120,274
Allowance for covered loans	9,492		13,528
Total allowance for loan losses	129,247	(129,854)	133,802	135,362	(138,626)	(124,704)
Allowance for unfunded letters of credit	2,350		2,383
Total allowance for credit losses	$ 131,597		$ 136,185