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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Jun. 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 | $ 706,468 | $ 773,932
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (8,671) | (13,175)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 92,573 | 85,152
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (8,997) | (9,326)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 799,041 | 859,084
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (17,668) | (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 | 5,825 | 4,898
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (20) | (11)
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 | 5,825 | 4,898
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (20) | (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 | 58,053 | 33,883
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (727) | (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 | 58,053 | 33,883
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (727) | (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 | 634,704 | 670,708
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (7,786) | (11,630)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 63,386 | 56,200
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (1,804) | (2,202)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 698,090 | 726,908
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (9,590) | (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 |  | 14,534
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 142
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 |  | 14,534
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | 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 | 100 | 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 | 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 | 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 | 7,786 | 33,277
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (138) | (762)
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 | 7,786 | 33,277
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (138) | (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 | 41.00% |
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 | 29,187 | 28,952
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (7,193) | (7,124)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 29,187 | 28,952
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (7,193) | $ (7,124)

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