Compensation and Benefit Plans - Summary Of Pension Plan Funded Status and Amounts Recognized in Financial Condition Statements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Pension Plans And Defined Benefit Postretirement Plans Disclosure [Abstract]
Projected benefit obligation	$ (15,883)	$ (14,142)	$ (14,903)	$ (15,883)	$ (14,142)
Fair value of plan assets	14,732	15,038	14,553	14,732	15,038
Funded status				(1,151)	896
Accumulated benefit obligation				$ (15,883)	$ (14,142)
Changes in benefit obligation:
Beginning of period	14,142	14,903
Service cost	39	39	35
Interest cost	581	615	584
2017 interest rate change	1,338
2017 mortality change	1,906
(Gain)/ Loss	(1,345)	(523)
Administrative cost	(39)	(39)
Benefits paid	(739)	(853)
End of period	15,883	14,142	14,903
Changes in plan assets:
Fair value of plan assets, beginning of year	15,038	14,553
Actual return on plan assets	472	507
Employer contributions		870
Benefits paid	(739)	(853)
Administrative expenses paid	(39)	(39)
Fair value of plan assets, end of year	$ 14,732	$ 15,038	$ 14,553