Cash and Cash Equivalents Securities Purchased Under Agreements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure Securities Purchased Under Agreements [Abstract]
Average securities purchased under agreements to resell	$ 94,094	$ 90,362
Maximum amount outstanding at any month-end during the year	$ 377,073	$ 316,059