Securities Purchased Under Agreements (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Securities Purchased Under Agreements to Resell [Line Items]
Average securities purchased under agreements to resell	$ 110,291	$ 41,113
Maximum amount outstanding at any month-end during the year	$ 375,236	$ 152,603