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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Mar. 31, 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 | $ 787,398 | $ 773,932
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (11,298) | (13,175)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 95,465 | 85,152
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (8,962) | (9,326)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 882,863 | 859,084
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (20,260) | (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 | 6,014 | 4,898
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (26) | (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 | 6,014 | 4,898
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (26) | (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 | 33,494 | 33,883
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (700) | (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 | 33,494 | 33,883
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (700) | (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 | 689,399 | 670,708
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (10,015) | (11,630)
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 66,191 | 56,200
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (2,008) | (2,202)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 755,590 | 726,908
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (12,023) | (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 | 14,485 | 14,534
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (51) | 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 | 14,485 | 14,534
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (51) | 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 | 16,642 | 16,632
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (58) | (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 | 16,642 | 16,632
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (58) | (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 | 22,180 | 33,277
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (443) | (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 | 22,180 | 33,277
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (443) | (762)
Foreign Government Debt Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 198 |
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (2) |
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 | 198 |
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | (2) |
Corporate Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 4,986 |
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (3) |
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 | 4,986 |
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (3) |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Percent of Unrealized Losses | 34.00% |
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 | 29,274 | 28,952
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | (6,954) | (7,124)
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 29,274 | 28,952
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (6,954) | $ (7,124)

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