Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 31,906	$ 278,207
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(18)	(3,121)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	546,617	733,222
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(5,631)	(24,933)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	578,523	1,011,429
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,649)	(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		2,289
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(41)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		5,028
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(140)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total		7,317
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(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	0	95,826
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(925)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	77,946	75,959
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(725)	(3,505)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	77,946	171,785
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(725)	(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	31,906	156,732
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(18)	(1,856)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	468,157	626,003
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4,905)	(20,789)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	500,063	782,735
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	(4,923)	(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
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		12,901
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(258)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total		12,901
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(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	514	9,338
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	(234)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total	514	17,929
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1)	(319)
Corporate Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		14,769
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		(214)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		3,993
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(7)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total		18,762
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ (221)