Securities - Trading Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 7,150	$ 4,858
Corporate Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	5,125	3,312
Mutual Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	1,499	1,037
Corporate Notes and Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	254	254
U.S. Government Sponsored Entities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	52	53
Certificates of Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities	$ 220	$ 202