Loans to Officers and Directors - Activity with Respect to Aggregate Loans to Related Parties (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Related Party Transactions [Abstract]
Balance – beginning of year	$ 321,392	$ 710,085
New loans	61,697	125,868
Repayments	(94,378)	(13,148)
Reduction due to change in reportable loans	(101,049)	(501,413)
Balance – end of year	$ 187,662	$ 321,392