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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Investment Securities [Line Items] |  |  |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost |  | $ 4,099 |
Amortized Cost |  | 872,153 | $ 740,846
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 2,573 | 4,854
Gross Unrealized Losses |  | (23,483) | (14,342)
Securities available for sale, at fair value |  | 851,243 | 731,358
Due within one year, Estimated Fair Value |  | 4,078 |
Due after one but within five years, Amortized Cost |  | 15,728 |
Due after one but within five years, Estimated Fair Value |  | 15,443 |
Due after five but within ten years, Amortized Cost |  | 26,339 |
Due after five but within ten years, Estimated Fair Value |  | 26,416 |
Due after ten years, Amortized Cost |  | 1,923 |
Due after ten years, Estimated Fair Value |  | 2,420 |
Due within one year, Amortized Cost |  | 48,089 |
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value |  | 48,357 |
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost | [1] | 822,394 |
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value | [1] | 801,216 |
US Government Agencies Residential Mortgage-backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 131,700 |
Securities available for sale, at fair value |  | 130,400 |
Residential Mortgage Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 670,752 | 625,555
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 100 | 1,097
Obligations of States and Political Subdivisions [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 27,168 | 27,410
Corporate Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  | 21,089 | 16,493
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |
Investment Securities [Line Items] |  |  |
Securities available for sale, at fair value |  |  | 23,646
Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 870,483 |
Securities available for sale, at fair value |  | 849,573 |
US Government Agency Debt Securities Commercial [Domain] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 121,996 | 24,611
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 36 | 0
Gross Unrealized Losses |  | (1,747) | (462)
Securities available for sale, at fair value |  | 120,285 | 24,149
Equity Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 1,670 | 1,670
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 0 | 0
Gross Unrealized Losses |  | 0 | 0
Securities available for sale, at fair value |  | 1,670 | 1,670
Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 870,483 | 739,176
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 2,573 | 4,854
Gross Unrealized Losses |  | (23,483) | (14,342)
Securities available for sale, at fair value |  | 849,573 | 729,688
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 0 | 27,499
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 0 | 526
Gross Unrealized Losses |  | 0 | (4,379)
Securities available for sale, at fair value |  | 0 | 23,646
Corporate Debt Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 20,902 | 15,907
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 497 | 590
Gross Unrealized Losses |  | (310) | (4)
Securities available for sale, at fair value |  | 21,089 | 16,493
Obligations of States and Political Subdivisions [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 27,087 | 27,083
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 120 | 327
Gross Unrealized Losses |  | (39) | 0
Securities available for sale, at fair value |  | 27,168 | 27,410
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 100 | 1,098
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 0 | 0
Gross Unrealized Losses |  | 0 | (1)
Securities available for sale, at fair value |  | 100 | 1,097
Residential Mortgage Backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 690,692 | 632,422
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 1,344 | 2,622
Gross Unrealized Losses |  | (21,284) | (9,489)
Securities available for sale, at fair value |  | 670,752 | 625,555
US Government Agencies Residential Mortgage-backed Securities [Member] |  |  |
Investment Securities [Line Items] |  |  |
Amortized Cost |  | 9,706 | 10,556
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 576 | 789
Gross Unrealized Losses |  | (103) | (7)
Securities available for sale, at fair value |  | $ 10,179 | $ 11,338
[1] | Mortgage-Backed Securities include an amortized cost of $131.7 million and a fair value of $130.4 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $690.7 million and a fair value of $670.8 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.

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