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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Mar. 31, 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 | $ 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)

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