Summary of Cash and Due from Banks (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Cash and Cash Equivalents [Line Items]
Due from the Federal Reserve	$ 144,135		$ 187,443
Cash and due from banks	397,339	446,580	383,757
Cash and due from banks at end of period	$ 541,474	$ 1,459,631	$ 571,200	$ 1,033,617