Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 627,488	$ 431,247
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(13,501)	(3,386)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	571,495	456,539
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(30,486)	(15,148)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	1,198,983	887,786
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(43,987)	(18,534)
US Government Agencies Residential Mortgage-backed Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	7,267	5,584
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(288)	(21)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	7,267	5,584
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(288)	(21)
US Government Agency Debt Securities Commercial [Domain]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	144,211	48,322
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,538)	(962)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	76,183	32,683
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3,879)	(894)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	220,394	81,005
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,417)	(1,856)
Residential Mortgage Backed Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	399,505	351,222
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(10,329)	(2,295)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	489,431	400,984
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(26,218)	(9,746)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	888,936	752,206
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(36,547)	(12,041)
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	12,986	13,985
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(427)	(71)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	12,986	13,985
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(427)	(71)
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	997
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(1)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	100	99
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	100	1,096
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	(1)
Obligations of States and Political Subdivisions [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	44,422	7,144
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(635)	(32)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,781	3,653
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(389)	(129)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	50,203	10,797
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,024)	(161)
Debt Security, Government, Non-US [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	395
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(5)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	395
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5)
Corporate Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	18,702	3,993
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(279)	(4)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	18,702	3,993
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (279)	(4)
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		0
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		19,120
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(4,379)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total		19,120
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ (4,379)