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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
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Debt and Equity Securities, FV-NI [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 117,209 | $ 278,207
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (348) | (3,121)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 257,981 | 733,222
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (2,655) | (24,933)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 375,190 | 1,011,429
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (3,003) | (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 | 18,120 | 75,959
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (106) | (3,505)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 18,120 | 171,785
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (106) | (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 | 115,615 | 156,732
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (342) | (1,856)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 239,861 | 626,003
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (2,549) | (20,789)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 355,476 | 782,735
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (2,891) | (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 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 | 998 |
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (2) |
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 | 998 |
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (2) |
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 |  | 8,591
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (85)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value |  | 9,338
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (234)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total |  | 17,929
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | (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 | 596 |
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (4) |
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 | 596 |
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (4) |
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)

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