Securities - Schedule of Unrealized Losses on Investment Securities (Detail) $ in Thousands	Jun. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 42,613	$ 466,878
Less than 12 months, Unrealized Losses	$ (233)	$ (3,372)
Less than 12 months, Number of Securities | Security	13	284
12 months or more, Fair Value	$ 660,179	$ 1,425,284
12 months or more, Unrealized Losses	$ (6,593)	$ (39,600)
12 months or more, Number of Securities | Security	307	837
Fair Value, Total	$ 702,792	$ 1,892,162
Unrealized Losses, Total	$ (6,826)	$ (42,972)
Number of Securities Total | Security	320	1,121
US Treasury Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value		$ 9,972
Less than 12 months, Unrealized Losses		$ (7)
Less than 12 months, Number of Securities | Security		1
Fair Value, Total		$ 9,972
Unrealized Losses, Total		$ (7)
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		$ 18,926
Less than 12 months, Unrealized Losses		$ (148)
Less than 12 months, Number of Securities | Security		8
12 months or more, Fair Value	$ 28,542	$ 76,385
12 months or more, Unrealized Losses	$ (242)	$ (1,937)
12 months or more, Number of Securities | Security	7	14
Fair Value, Total	$ 28,542	$ 95,311
Unrealized Losses, Total	$ (242)	$ (2,085)
Number of Securities Total | Security	7	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	$ 30,301	$ 285,534
Less than 12 months, Unrealized Losses	$ (95)	$ (1,862)
Less than 12 months, Number of Securities | Security	4	44
12 months or more, Fair Value	$ 564,036	$ 922,698
12 months or more, Unrealized Losses	$ (5,988)	$ (29,829)
12 months or more, Number of Securities | Security	211	291
Fair Value, Total	$ 594,337	$ 1,208,232
Unrealized Losses, Total	$ (6,083)	$ (31,691)
Number of Securities Total | Security	215	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	$ 3,783	$ 9,186
Less than 12 months, Unrealized Losses	$ (26)	$ (18)
Less than 12 months, Number of Securities | Security	4	6
12 months or more, Fair Value	$ 26,655	$ 111,068
12 months or more, Unrealized Losses	$ (90)	$ (2,945)
12 months or more, Number of Securities | Security	12	14
Fair Value, Total	$ 30,438	$ 120,254
Unrealized Losses, Total	$ (116)	$ (2,963)
Number of Securities Total | Security	16	20
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 609	$ 104,469
Less than 12 months, Unrealized Losses	$ (1)	$ (439)
Less than 12 months, Number of Securities | Security	2	207
12 months or more, Fair Value	$ 38,999	$ 303,681
12 months or more, Unrealized Losses	$ (230)	$ (4,555)
12 months or more, Number of Securities | Security	76	513
Fair Value, Total	$ 39,608	$ 408,150
Unrealized Losses, Total	$ (231)	$ (4,994)
Number of Securities Total | Security	78	720
Corporate Debt Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 7,920	$ 38,791
Less than 12 months, Unrealized Losses	$ (111)	$ (898)
Less than 12 months, Number of Securities | Security	3	18
12 months or more, Fair Value	$ 1,947	$ 11,452
12 months or more, Unrealized Losses	$ (43)	$ (334)
12 months or more, Number of Securities | Security	1	5
Fair Value, Total	$ 9,867	$ 50,243
Unrealized Losses, Total	$ (154)	$ (1,232)
Number of Securities Total | Security	4	23