Securities - Schedule of Unrealized Losses on Investment Securities (Detail) $ in Thousands	Sep. 30, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 376,947	$ 466,878
Less than 12 months, Unrealized Losses	$ (1,609)	$ (3,372)
Less than 12 months, Number of Securities | Security	98	284
12 months or more, Fair Value	$ 332,683	$ 1,425,284
12 months or more, Unrealized Losses	$ (3,362)	$ (39,600)
12 months or more, Number of Securities | Security	152	837
Fair Value, Total	$ 709,630	$ 1,892,162
Unrealized Losses, Total	$ (4,971)	$ (42,972)
Number of Securities Total | Security	250	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	$ 5,383	$ 18,926
Less than 12 months, Unrealized Losses	$ (17)	$ (148)
Less than 12 months, Number of Securities | Security	4	8
12 months or more, Fair Value	$ 6,781	$ 76,385
12 months or more, Unrealized Losses	$ (108)	$ (1,937)
12 months or more, Number of Securities | Security	2	14
Fair Value, Total	$ 12,164	$ 95,311
Unrealized Losses, Total	$ (125)	$ (2,085)
Number of Securities Total | Security	6	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	$ 347,848	$ 285,534
Less than 12 months, Unrealized Losses	$ (1,515)	$ (1,862)
Less than 12 months, Number of Securities | Security	70	44
12 months or more, Fair Value	$ 306,696	$ 922,698
12 months or more, Unrealized Losses	$ (2,986)	$ (29,829)
12 months or more, Number of Securities | Security	128	291
Fair Value, Total	$ 654,544	$ 1,208,232
Unrealized Losses, Total	$ (4,501)	$ (31,691)
Number of Securities Total | Security	198	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	$ 11,066	$ 9,186
Less than 12 months, Unrealized Losses	$ (38)	$ (18)
Less than 12 months, Number of Securities | Security	3	6
12 months or more, Fair Value	$ 3,729	$ 111,068
12 months or more, Unrealized Losses	$ (7)	$ (2,945)
12 months or more, Number of Securities | Security	2	14
Fair Value, Total	$ 14,795	$ 120,254
Unrealized Losses, Total	$ (45)	$ (2,963)
Number of Securities Total | Security	5	20
Obligations of State and Political Subdivisions [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value	$ 12,650	$ 104,469
Less than 12 months, Unrealized Losses	$ (39)	$ (439)
Less than 12 months, Number of Securities | Security	21	207
12 months or more, Fair Value	$ 7,581	$ 303,681
12 months or more, Unrealized Losses	$ (132)	$ (4,555)
12 months or more, Number of Securities | Security	17	513
Fair Value, Total	$ 20,231	$ 408,150
Unrealized Losses, Total	$ (171)	$ (4,994)
Number of Securities Total | Security	38	720
Corporate Debt Securities [Member]
Net Unrealized Gains And Losses On Investments [Line Items]
Less than 12 months, Fair Value		$ 38,791
Less than 12 months, Unrealized Losses		$ (898)
Less than 12 months, Number of Securities | Security		18
12 months or more, Fair Value	$ 7,896	$ 11,452
12 months or more, Unrealized Losses	$ (129)	$ (334)
12 months or more, Number of Securities | Security	3	5
Fair Value, Total	$ 7,896	$ 50,243
Unrealized Losses, Total	$ (129)	$ (1,232)
Number of Securities Total | Security	3	23