Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 48,478	$ 278,207
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(28)	(3,121)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	865,485	733,222
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(15,586)	(24,933)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	913,963	1,011,429
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(15,614)	(28,054)
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	0	2,289
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(41)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,336	5,028
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(50)	(140)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	7,336	7,317
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(50)	(181)
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	48,274	95,826
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(28)	(925)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	124,003	75,959
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(3,225)	(3,505)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	172,277	171,785
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,253)	(4,430)
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	4	156,732
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(1,856)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	710,446	626,003
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(12,166)	(20,789)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	710,450	782,735
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(12,166)	(22,645)
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	0	0
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12,799	12,901
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(95)	(258)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	12,799	12,901
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(95)	(258)
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	0	8,591
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(85)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,925	9,338
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(31)	(234)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	5,925	17,929
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(31)	(319)
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	200
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	0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	200
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	0
Corporate Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	14,769
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(214)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,976	3,993
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(19)	(7)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	4,976	18,762
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (19)	$ (221)