Fair Value of Assets and Liabilities - Roll-Forward of the Consolidated Condensed Statement of Financial Condition Items (Detail) - Other Assets [Member] - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, Beginning balance	$ 1,817	$ 1,865
Change in unrealized gain included in other comprehensive income for assets and liabilities still held at June 30,	40	97
Accretion of previously discounted amounts	8	16
Purchases, issuances and settlements, net	0	0
Transfers in and/or out of Level 3	0	0
Fair value, Ending balance	$ 1,865	$ 1,978