Securities - Gross Unrealized Losses, Aggregated by Investment Category and Length of Time (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	$ (4,637)	$ (140)
Fair Value, Less Than Twelve Months	213,054	51,893
Gross Unrealized Losses, Total	(4,637)	(140)
Fair Value, Total	213,054	51,893
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(734)
Fair Value, Less Than Twelve Months	43,325
Gross Unrealized Losses, Total	(734)
Fair Value, Total	43,325
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(1,998)	(130)
Fair Value, Less Than Twelve Months	94,069	48,331
Gross Unrealized Losses, Total	(1,998)	(130)
Fair Value, Total	94,069	48,331
Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(1,380)
Fair Value, Less Than Twelve Months	57,940
Gross Unrealized Losses, Total	(1,380)
Fair Value, Total	57,940
State and Local Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses, Less Than Twelve Months	(525)	(10)
Fair Value, Less Than Twelve Months	17,720	3,562
Gross Unrealized Losses, Total	(525)	(10)
Fair Value, Total	$ 17,720	$ 3,562