Securities - Schedule of Unrealized Losses on Investment Securities (Detail) $ in Thousands	Dec. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 340,022	$ 668,960
Less than 12 months, Unrealized Losses	$ (1,638)	$ (4,290)
Less than 12 months, Number of Securities | Security	53	172
12 months or more, Fair Value	$ 4,136	$ 292,520
12 months or more, Unrealized Losses	$ (121)	$ (3,066)
12 months or more, Number of Securities | Security	3	136
Fair Value, Total	$ 344,158	$ 961,480
Unrealized Losses, Total	$ (1,759)	$ (7,356)
Number of Securities Total | Security	56	308
US Treasury Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value		$ 1,499
Less than 12 months, Unrealized Losses		$ (1)
Less than 12 months, Number of Securities | Security		1
Fair Value, Total		$ 1,499
Unrealized Losses, Total		$ (1)
Number of Securities Total | Security		1
U.S. Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 18,308	$ 57,650
Less than 12 months, Unrealized Losses	$ (142)	$ (274)
Less than 12 months, Number of Securities | Security	2	25
12 months or more, Fair Value		$ 6,593
12 months or more, Unrealized Losses		$ (164)
12 months or more, Number of Securities | Security		2
Fair Value, Total	$ 18,308	$ 64,243
Unrealized Losses, Total	$ (142)	$ (438)
Number of Securities Total | Security	2	27
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 224,448	$ 544,692
Less than 12 months, Unrealized Losses	$ (1,227)	$ (3,725)
Less than 12 months, Number of Securities | Security	41	116
12 months or more, Fair Value	$ 4,136	$ 272,884
12 months or more, Unrealized Losses	$ (121)	$ (2,667)
12 months or more, Number of Securities | Security	3	122
Fair Value, Total	$ 228,584	$ 817,576
Unrealized Losses, Total	$ (1,348)	$ (6,392)
Number of Securities Total | Security	44	238
Commercial Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 97,266	$ 43,123
Less than 12 months, Unrealized Losses	$ (269)	$ (124)
Less than 12 months, Number of Securities | Security	10	7
12 months or more, Fair Value		$ 3,704
12 months or more, Unrealized Losses		$ (5)
12 months or more, Number of Securities | Security		2
Fair Value, Total	$ 97,266	$ 46,827
Unrealized Losses, Total	$ (269)	$ (129)
Number of Securities Total | Security	10	9
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value		$ 17,876
Less than 12 months, Unrealized Losses		$ (122)
Less than 12 months, Number of Securities | Security		22
12 months or more, Fair Value		$ 4,413
12 months or more, Unrealized Losses		$ (137)
12 months or more, Number of Securities | Security		8
Fair Value, Total		$ 22,289
Unrealized Losses, Total		$ (259)
Number of Securities Total | Security		30
Corporate Debt Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value		$ 4,120
Less than 12 months, Unrealized Losses		$ (44)
Less than 12 months, Number of Securities | Security		1
12 months or more, Fair Value		$ 4,926
12 months or more, Unrealized Losses		$ (93)
12 months or more, Number of Securities | Security		2
Fair Value, Total		$ 9,046
Unrealized Losses, Total		$ (137)
Number of Securities Total | Security		3