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Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 USD ($) Bank | Dec. 31, 2016 USD ($)
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | $ 740,846 | $ 790,082
Estimated Fair Value of Available for Sale | 731,358 | $ 778,612
Mezzanine [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | 27,499 |
Estimated Fair Value of Available for Sale | 23,646 |
Unrealized Gain (Loss) | (3,853) |
Mezzanine [Member] | Pre TSL IV [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | 1,817 |
Estimated Fair Value of Available for Sale | 1,405 |
Unrealized Gain (Loss) | $ (412) |
Debt Instrument, Credit Rating | Ba1/BB |
Number of Banks | Bank | 6 |
Deferrals and Defaults as a % of Current Collateral | 18.05% |
Excess Subordination as a % of Current Performing Collateral | 72.13% |
Mezzanine [Member] | Pre TSL VIII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | $ 2,043 |
Estimated Fair Value of Available for Sale | 2,228 |
Unrealized Gain (Loss) | $ 185 |
Debt Instrument, Credit Rating | C/C |
Number of Banks | Bank | 26 |
Deferrals and Defaults as a % of Current Collateral | 38.52% |
Excess Subordination as a % of Current Performing Collateral | 0.00% |
Mezzanine [Member] | Pre TSL IX [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | $ 2,448 |
Estimated Fair Value of Available for Sale | 2,052 |
Unrealized Gain (Loss) | $ (396) |
Debt Instrument, Credit Rating | B1/C |
Number of Banks | Bank | 37 |
Deferrals and Defaults as a % of Current Collateral | 27.83% |
Excess Subordination as a % of Current Performing Collateral | 19.46% |
Mezzanine [Member] | Pre TSL X [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | $ 1,863 |
Estimated Fair Value of Available for Sale | 2,125 |
Unrealized Gain (Loss) | $ 262 |
Debt Instrument, Credit Rating | Caa1/C |
Number of Banks | Bank | 41 |
Deferrals and Defaults as a % of Current Collateral | 27.93% |
Excess Subordination as a % of Current Performing Collateral | 1.62% |
Mezzanine [Member] | Pre TSL XII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | $ 6,097 |
Estimated Fair Value of Available for Sale | 5,209 |
Unrealized Gain (Loss) | $ (888) |
Debt Instrument, Credit Rating | B3/C |
Number of Banks | Bank | 63 |
Deferrals and Defaults as a % of Current Collateral | 23.35% |
Excess Subordination as a % of Current Performing Collateral | 0.00% |
Mezzanine [Member] | Pre TSL XIV [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | $ 13,136 |
Estimated Fair Value of Available for Sale | 10,453 |
Unrealized Gain (Loss) | $ (2,683) |
Debt Instrument, Credit Rating | Ba2/CCC |
Number of Banks | Bank | 49 |
Deferrals and Defaults as a % of Current Collateral | 12.95% |
Excess Subordination as a % of Current Performing Collateral | 39.26% |
Mezzanine [Member] | MMCap I [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Amortized Cost | $ 95 |
Estimated Fair Value of Available for Sale | 174 |
Unrealized Gain (Loss) | $ 79 |
Debt Instrument, Credit Rating | Ca/C |
Number of 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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