Compensation and Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount Recognized In Net Periodic Benefit Cost And Other Comprehensive Income Loss Before Tax [Abstract]
Service cost	$ 39	$ 39	$ 35
Interest cost	581	615	584
Expected return on plan assets	(839)	(848)	(818)
Amortization of prior service cost	25	25	25
Amortization of (gain)/loss	234	248	238
Net periodic benefit cost	$ 40	$ 79	$ 64