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Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) Bank | Dec. 31, 2017 USD ($)
Impairment Of Investment Securities [Line Items] |  |
Securities available for sale, at fair value | $ 812,877 | $ 731,358
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | 4,474 | 4,854
Unrealized Gain (Loss) | 20,785 | $ 14,342
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 13,602 |
Securities available for sale, at fair value | 14,132 |
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | 530 |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IV [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 933 |
Securities available for sale, at fair value | 738 |
Unrealized Gain (Loss) | $ 195 |
Debt Instrument, Credit Rating | Ba1/BB |
Number of Banks (in banks) | Bank | 5 |
Deferrals and Defaults as a % of Current Collateral | 0.00% |
Excess Subordination as a % of Current Performing Collateral | 113.79% |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VIII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | $ 2,072 |
Securities available for sale, at fair value | 2,295 |
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | $ 223 |
Debt Instrument, Credit Rating | C/C |
Number of Banks (in banks) | Bank | 26 |
Deferrals and Defaults as a % of Current Collateral | 38.52% |
Excess Subordination as a % of Current Performing Collateral | 0.00% |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IX [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | $ 2,457 |
Securities available for sale, at fair value | 3,000 |
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | $ 543 |
Debt Instrument, Credit Rating | B1/C |
Number of Banks (in banks) | Bank | 37 |
Deferrals and Defaults as a % of Current Collateral | 27.83% |
Excess Subordination as a % of Current Performing Collateral | 19.46% |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL X [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | $ 1,894 |
Securities available for sale, at fair value | 2,301 |
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | $ 407 |
Debt Instrument, Credit Rating | Caa1/C |
Number of Banks (in banks) | Bank | 41 |
Deferrals and Defaults as a % of Current Collateral | 26.29% |
Excess Subordination as a % of Current Performing Collateral | 1.73% |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | $ 6,151 |
Securities available for sale, at fair value | 5,624 |
Unrealized Gain (Loss) | $ 527 |
Debt Instrument, Credit Rating | B3/C |
Number of Banks (in banks) | Bank | 64 |
Deferrals and Defaults as a % of Current Collateral | 23.39% |
Excess Subordination as a % of Current Performing Collateral | 0.00% |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | MMCap I [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | $ 95 |
Securities available for sale, at fair value | 174 |
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | $ 79 |
Debt Instrument, Credit Rating | Ca/C |
Number of Banks (in banks) | Bank | 7 |
Deferrals and Defaults as a % of Current Collateral | 69.35% |
Excess Subordination as a % of Current Performing Collateral | 69.99% |

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