Securities Purchased Under Agreements (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Disclosure Securities Purchased Under Agreements [Abstract]
Average securities purchased under agreements to resell	$ 131,287	$ 110,291
Maximum amount outstanding at any month-end during the year	$ 266,534	$ 375,236