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Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014
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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 | $ 97,754 | $ 229,583
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 306 | 694
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 398,971 | 523,463
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 11,929 | 23,726
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 496,725 | 753,046
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | 12,235 | 24,420
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 | 2,240 | 2,318
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 3
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 | 2,240 | 2,318
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | 0 | 3
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 | 78,300 | 111,646
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 242 | 419
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 277,333 | 368,706
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 4,610 | 9,411
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 355,633 | 480,352
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | 4,852 | 9,830
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 | 10,091 | 112,473
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 9 | 229
Investment Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | 91,570 | 130,401
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 82 | 1,079
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 101,661 | 242,874
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 91 | 1,308
Obligations of States and Political Subdivisions [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Percent of Unrealized Losses | 1.00% |
Investment Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | $ 7,123 | 3,146
Investment Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 55 | 43
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 | 7,123 | 3,146
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 55 | 43
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Percent of Unrealized Losses | 59.00% |
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 | 30,068 | 24,356
Investment Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 7,237 | 13,236
Investment Securities, Continuous Unrealized Loss Position, Fair Value, Total | 30,068 | 24,356
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ 7,237 | $ 13,236

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