Securities - Schedule of Unrealized Losses on Investment Securities (Detail) $ in Thousands	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 668,960	$ 466,878
Less than 12 months, Unrealized Losses	$ (4,290)	$ (3,372)
Less than 12 months, Number of Securities | Security	172	284
12 months or more, Fair Value	$ 292,520	$ 1,425,284
12 months or more, Unrealized Losses	$ (3,066)	$ (39,600)
12 months or more, Number of Securities | Security	136	837
Fair Value, Total	$ 961,480	$ 1,892,162
Unrealized Losses, Total	$ (7,356)	$ (42,972)
Number of Securities Total | Security	308	1,121
US Treasury Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 1,499	$ 9,972
Less than 12 months, Unrealized Losses	$ (1)	$ (7)
Less than 12 months, Number of Securities | Security	1	1
Fair Value, Total	$ 1,499	$ 9,972
Unrealized Losses, Total	$ (1)	$ (7)
Number of Securities Total | Security	1	1
U.S. Government Sponsored Entities and Agencies [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 57,650	$ 18,926
Less than 12 months, Unrealized Losses	$ (274)	$ (148)
Less than 12 months, Number of Securities | Security	25	8
12 months or more, Fair Value	$ 6,593	$ 76,385
12 months or more, Unrealized Losses	$ (164)	$ (1,937)
12 months or more, Number of Securities | Security	2	14
Fair Value, Total	$ 64,243	$ 95,311
Unrealized Losses, Total	$ (438)	$ (2,085)
Number of Securities Total | Security	27	22
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	$ 544,692	$ 285,534
Less than 12 months, Unrealized Losses	$ (3,725)	$ (1,862)
Less than 12 months, Number of Securities | Security	116	44
12 months or more, Fair Value	$ 272,884	$ 922,698
12 months or more, Unrealized Losses	$ (2,667)	$ (29,829)
12 months or more, Number of Securities | Security	122	291
Fair Value, Total	$ 817,576	$ 1,208,232
Unrealized Losses, Total	$ (6,392)	$ (31,691)
Number of Securities Total | Security	238	335
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	$ 43,123	$ 9,186
Less than 12 months, Unrealized Losses	$ (124)	$ (18)
Less than 12 months, Number of Securities | Security	7	6
12 months or more, Fair Value	$ 3,704	$ 111,068
12 months or more, Unrealized Losses	$ (5)	$ (2,945)
12 months or more, Number of Securities | Security	2	14
Fair Value, Total	$ 46,827	$ 120,254
Unrealized Losses, Total	$ (129)	$ (2,963)
Number of Securities Total | Security	9	20
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 17,876	$ 104,469
Less than 12 months, Unrealized Losses	$ (122)	$ (439)
Less than 12 months, Number of Securities | Security	22	207
12 months or more, Fair Value	$ 4,413	$ 303,681
12 months or more, Unrealized Losses	$ (137)	$ (4,555)
12 months or more, Number of Securities | Security	8	513
Fair Value, Total	$ 22,289	$ 408,150
Unrealized Losses, Total	$ (259)	$ (4,994)
Number of Securities Total | Security	30	720
Corporate Debt Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 4,120	$ 38,791
Less than 12 months, Unrealized Losses	$ (44)	$ (898)
Less than 12 months, Number of Securities | Security	1	18
12 months or more, Fair Value	$ 4,926	$ 11,452
12 months or more, Unrealized Losses	$ (93)	$ (334)
12 months or more, Number of Securities | Security	2	5
Fair Value, Total	$ 9,046	$ 50,243
Unrealized Losses, Total	$ (137)	$ (1,232)
Number of Securities Total | Security	3	23