Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Dec. 31, 2002	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2019
Property, Plant and Equipment [Line Items]
Cash and cash equivalents maturity period, days		90 days
Percentage of participants matched contribution		6.00%
Dividends paid on unallocated ESOP shares over period	10 years
Dividends paid on unallocated ESOP shares over period		30 years
Shares held by the DDFP			219,281
Operating lease, right-of-use asset		$ 41,754		$ 44,900
Present value of net future minimum lease payments		$ 42,815		46,100
ASU 2016-02
Property, Plant and Equipment [Line Items]
Operating lease, right-of-use asset				44,900
Present value of net future minimum lease payments				$ 46,100
Depositor Relationships
Property, Plant and Equipment [Line Items]
Amortized accelerated basis period		8 years 9 months 18 days
Beacon Trust | Customer Relationships
Property, Plant and Equipment [Line Items]
Amortized accelerated basis period		12 years
The MDE Group | Customer Relationships
Property, Plant and Equipment [Line Items]
Amortized accelerated basis period		10 years 4 months 24 days
Tirschwell & Loewy, Inc. | Customer Relationships
Property, Plant and Equipment [Line Items]
Amortized accelerated basis period		10 years