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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Jun. 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 | $ 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)

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