Securities - Schedule of Unrealized Losses on Investment Securities (Detail) $ in Thousands	Mar. 31, 2021 USD ($) Security	Dec. 31, 2020 USD ($) Security
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 1,128,184	$ 340,022
Less than 12 months, Unrealized Losses	$ (17,342)	$ (1,638)
Less than 12 months, Number of Securities | Security	114	53
12 months or more, Fair Value	$ 3,531	$ 4,136
12 months or more, Unrealized Losses	$ (82)	$ (121)
12 months or more, Number of Securities | Security	4	3
Fair Value, Total	$ 1,131,715	$ 344,158
Unrealized Losses, Total	$ (17,424)	$ (1,759)
Number of Securities Total | Security	118	56
U.S. Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 56,270	$ 18,308
Less than 12 months, Unrealized Losses	$ (2,706)	$ (142)
Less than 12 months, Number of Securities | Security	6	2
Fair Value, Total	$ 56,270	$ 18,308
Unrealized Losses, Total	$ (2,706)	$ (142)
Number of Securities Total | Security	6	2
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	$ 973,961	$ 224,448
Less than 12 months, Unrealized Losses	$ (13,879)	$ (1,227)
Less than 12 months, Number of Securities | Security	98	41
12 months or more, Fair Value	$ 3,531	$ 4,136
12 months or more, Unrealized Losses	$ (82)	$ (121)
12 months or more, Number of Securities | Security	4	3
Fair Value, Total	$ 977,492	$ 228,584
Unrealized Losses, Total	$ (13,961)	$ (1,348)
Number of Securities Total | Security	102	44
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	$ 94,186	$ 97,266
Less than 12 months, Unrealized Losses	$ (721)	$ (269)
Less than 12 months, Number of Securities | Security	9	10
Fair Value, Total	$ 94,186	$ 97,266
Unrealized Losses, Total	$ (721)	$ (269)
Number of Securities Total | Security	9	10
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 3,767
Less than 12 months, Unrealized Losses	$ (36)
Less than 12 months, Number of Securities | Security	1
Fair Value, Total	$ 3,767
Unrealized Losses, Total	$ (36)
Number of Securities Total | Security	1