Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Jun. 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	$ 665,488	$ 431,247
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(16,172)	(3,386)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	418,470	456,539
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(20,222)	(15,148)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	1,083,958	887,786
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(36,394)	(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	7,106	5,584
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(231)	(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	7,106	5,584
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(231)	(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	71,408	48,322
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,747)	(962)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	54,482	32,683
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,509)	(894)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	125,890	81,005
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4,256)	(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	523,619	351,222
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(13,093)	(2,295)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	360,389	400,984
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(17,432)	(9,746)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	884,008	752,206
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(30,525)	(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	13,290	13,985
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(369)	(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	13,290	13,985
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(369)	(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	31,005	7,144
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(413)	(32)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3,499	3,653
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(281)	(129)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	34,504	10,797
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(694)	(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	391
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(9)
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	391
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(9)
Corporate Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	18,669	3,993
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(310)	(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	18,669	3,993
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (310)	(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)