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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Debt and Equity Securities, FV-NI [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 627,488 | $ 431,247
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (13,501) | (3,386)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 571,495 | 456,539
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (30,486) | (15,148)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 1,198,983 | 887,786
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (43,987) | (18,534)
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 | 7,267 | 5,584
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (288) | (21)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 7,267 | 5,584
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (288) | (21)
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 | 144,211 | 48,322
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (1,538) | (962)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 76,183 | 32,683
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (3,879) | (894)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 220,394 | 81,005
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (5,417) | (1,856)
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 | 399,505 | 351,222
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (10,329) | (2,295)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 489,431 | 400,984
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (26,218) | (9,746)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 888,936 | 752,206
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (36,547) | (12,041)
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 | 12,986 | 13,985
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (427) | (71)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 12,986 | 13,985
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (427) | (71)
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 | 0 | 997
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | (1)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 100 | 99
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 100 | 1,096
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | 0 | (1)
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 | 44,422 | 7,144
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (635) | (32)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 5,781 | 3,653
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (389) | (129)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 50,203 | 10,797
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (1,024) | (161)
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 | 395 |
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (5) |
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 | 395 |
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (5) |
Corporate Securities [Member] |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 18,702 | 3,993
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (279) | (4)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 18,702 | 3,993
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (279) | (4)
Pooled Trust Preferred Collateralized Debt Obligations [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 |  | 19,120
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (4,379)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total |  | 19,120
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | $ (4,379)

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