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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
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Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 478,939 | $ 773,932
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (3,650) | (13,175)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 259,307 | 85,152
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (11,601) | (9,326)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 738,246 | 859,084
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (15,251) | (22,501)
US Government Agencies Residential Mortgage-backed Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value |  | 4,898
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (11)
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 |  | 4,898
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | (11)
US Government Agency Debt Securities Commercial [Domain] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 82,134 | 33,883
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (1,078) | (561)
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 | 82,134 | 33,883
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (1,078) | (561)
Residential Mortgage Backed Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 377,725 | 670,708
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (2,526) | (11,630)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 226,562 | 56,200
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (5,033) | (2,202)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 604,287 | 726,908
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (7,559) | (13,832)
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 14,222 | 14,534
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (4) | 142
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 | 14,222 | 14,534
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (4) | 142
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 0 | 16,632
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | (69)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 100 | 0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 100 | 16,632
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | 0 | (69)
Obligations of States and Political Subdivisions [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 4,858 | 33,277
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (42) | (762)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 2,591 | 0
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (87) | 0
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 7,449 | 33,277
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (129) | (762)
Corporate Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 0 |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Percent of Unrealized Losses | 42.00% |
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 | 30,054 | 28,952
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (6,481) | (7,124)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 30,054 | 28,952
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (6,481) | $ (7,124)

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