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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Sep. 30, 2016 | Dec. 31, 2015
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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 | $ 61,472 | $ 493,938
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (162) | (3,881)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 90,805 | 293,076
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (8,507) | (14,910)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 152,277 | 787,014
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (8,669) | (18,791)
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 |  | 6,798
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (20)
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 |  | 6,798
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | (20)
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 |  | 16,596
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (247)
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 |  | 16,596
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | (247)
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 | 59,691 | 436,011
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (137) | (3,293)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 60,909 | 263,119
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (419) | (7,413)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 120,600 | 699,130
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (556) | (10,706)
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 |  | 15,009
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | 178
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 |  | 15,009
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | 178
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 |  | 12,316
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (85)
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 |  | 12,316
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss |  | (85)
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 | 1,781 | 7,208
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (25) | (58)
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 | 1,781 | 7,208
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (25) | (58)
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Percent of Unrealized Losses | 93.00% |
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 | 29,896 | 29,957
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (8,088) | (7,497)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 29,896 | 29,957
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (8,088) | $ (7,497)

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